Easy Communication

In real life, a general cannot immediately give orders to a unit and have them react instantaneously. Messages have to be sent, colonels, majors and finally captains and corporals have to be reached, and finally the orders need to be received by the soldiers. This is doubly true for ancient times.

In strategy games, on the other hand, this doesn't happen. When you, the general, click the screen, the orders are instantly transmitted from thought to deed. Its as if there's some kind of psychic link from HQ to the unit receiving the order. Particularly jarring in ancient settings when a general can give orders to soldiers a town away. That general must have one hell of a loud voice, or a very good herald. Although, to be fair, militaries developed various methods to cope with this problem prior to the invention of radio telephony: all those trumpets, bugles, drums, fifes, flags, banners, and standards that modern militaries use while on parade were actually used for transmitting signals across pre-radio battlefields. Even so, this cannot explain the rapidity with which orders are carried out in most games. Some argue though, that if building a command center or a town hall in your average RTS takes 90 seconds tops, then order transmission times must have been scaled down by a similar factor.

Admittedly this comes under Acceptable Breaks From Reality, as it would be annoying to wait for your orders to filter down through the proper chain of command.

What is less forgivable is when the soldiers seem to be completely dependent on the player for orders, not responding even when under fire from a long ranged unit or even retreating/defending itself from its assailant. There are reasons for soldiers being designed this way: a unit could come under long-ranged fire, respond, and get lured into an ambush. But its still annoying to see your unit get whittled down to nearly no health because you happened to not be around to give the order and the unit just refuses to do anything to save itself.

The small-scale variation of this which pops up in Tactical Shooters is Squad Controls; generally more justifiable, as the squad tend to be within the PC's earshot/eyeline, but can raise questions if you're playing a Stealth Based Mission.

Closely related to Easy Logistics. Much like Command and Conquer Economy only this applies to units instead of buildings being dependent on the players orders. Usually, all three tropes will be present together.

This is so common in strategy games that it may be best to save examples for aversions or particularly blatant examples of the trope.

Blatant examples.

 * Total War is particularly vulnerable to Fridge Logic regarding this. A group of highly impetuous knights that are completely embroiled in a chaotic mix of friendly and hostile forces will, at the orders of a general half a battlefield away, break off, reform (a very difficult task for cavalry) and then can be ordered by the same general to circle round the enemy army and attack from the rear. The units in this game also don't respond when under missile attack, but in this case its justified as a group of infantry suddenly charging out to attack some archers would throw a players strategy out of kilter and possibly result in the loss of the battle.
 * The second half of the trope is subverted in the case of units with the Impetuous trait (usually knights and similar elite units), who can decide to charge the enemy on their own. They invariably do so when least convenient to the player - isn't it just swell when the linchpin of your defensive line leaves a gaping hole in it in order to charge some dirty peasants halfway across the field ? Truth in Television, too : that's pretty much how the French lost Crécy.
 * Some units will also refuse to break off while pursuing targets, which can wreck things if you need those Feudal Knights to rush over and guard your archers from enemy cavalry, or you need to get your Scottish Highlanders around the enemy to flank those soldiers who are winning the battle against your dismounted knights.
 * Age of Empires II had the defensive orders, which were useful if you wanted a unit to not stray too far away from a position. Unfortunately, some hostile units could fire further than the distance the defensive unit was scripted to respond to aggressively. The result was that the unit would just stand there and get peppered to death. (I don't know if other Age of Empires games suffered from this or not.)
 * If you order peasants to build a building, and they are fired upon while doing so, they'll walk alway until they're out of range, then return to try to construct the building again. If you order a dozen peasants to work on the same building, often they'll be able construct the building before they all die. But if there's only one or two peasants, often they'll just walk back and forth, getting shot at, without ever doing any work on the building, until they die.
 * Fire Emblem 7 makes the commander an entity to represent the player (but has a minor role and second playthoughs give the option to remove him entirely.) who can issue someone orders from any distance. The rest of the games simply make the player a Non-Entity General (though FE 4 has the oddity of units cross country coordinating)
 * Command and Conquer games have this in spades, if an enemy attacks from outside their detection range they'll just stand there slowly dying, without even thinking to move away. The Generals series added an auto-retaliation option that let players allow their units to attack enemies who engaged them from outside their sight range.
 * Being able to rapidly communicate with the soldiers, though, is actually justified in the setting by the EVA units relaying orders quickly and decisively to the troops in the field.
 * The first Red Alert had an interesting case: most units would not auto-acquire targets unless set to "guard", and would never auto-acquire a building (although this is so they don't destroy anything you wanted to steal). Worse, a particular elite unit, Tanya, could never be put into guard mode, and had to be directed for every shot - but, with her healthy range and high damage against infantry, she needed something to hinder her.
 * This will actually become extremely frustrating in one particular Allies mission, which is filled with fast-running dogs that come in packs. Hope you're a fast click or you'll be seeing that mushroom cloud quite a lot. Note that she cannot shoot through walls, no matter how close the enemy is.
 * Civilization has this in all its installations. A ship can be halfway across the ocean and then suddenly decide in mid-journey to change direction and visit an entirely different continent. HQ must have really efficient carrier pigeons in the ages before radio.
 * Mind you, a turn can represent anything between 1 to 60 years, so it's not entirely unreasonable to expect that some communication would occur in that time.
 * Well it doesn't make sense when you tell a unit to go out and explore only to recall them when they're halfway across the world.
 * Making this less a case of Easy Communication and more a case of Easy Logistics.
 * Well, there's also meeting with other world leaders on a whim (despite in some cases lacking the appropriate technology to go visit them) and news of world wonders being constructed in cities you've never heard of by civilizations you've never met.
 * Both blatantly displayed and averted in Evony. On one hand, you can apparently receive news from players miles away instantly. In the medieval world. On the other hand, armies take realistic amounts of time to travel from place to place. (A real headache for alliances whose members are not close together.)
 * Though not a strategy game, this is gratuitously played straight in the Warriors series (Dynasty Warriors et al), where hostile commanders can apparently have a real-time conversation from opposite ends of a raging battlefield.

Aversions

 * Praetorians plays this straight mostly, but if your units get embroiled in battle with another unit, you won't be able to give them orders until the hostile unit is dead.
 * Knights of Honor subverts the second half of this trope with your knights who will charge any unit within range, this can get grisly as I saw entire groups of knights falling to the pitchforks of rabble.
 * This is actually quite historically accurate in some medieval armies. Knights, particularly French knights, were notoriously impetuous, and several battles in history were lost due to undisciplined, glory-hungry knights insisting on being the first into battle. The French knights at Agincourt and Crecy were notorious for this.
 * Warhammer Dark Omen has the variant where you can command units normally, but once engaged in melee the only way to get them out is if one side or the other breaks. (although you can deliberately order a unit to break) which is generally a very bad idea considering that routing units take large losses and are nigh irreplaceable.
 * The squad sergeants in Full Spectrum Warrior respond quickly, getting their orders over radio, but there's still a delay as they need to verbally relay those orders to their fireteams before they move or shift fire.
 * Dominions probably comes as close to averting this as possible. Instead of commanding your soldiers in battle, you give orders (formation, battle plan, spells to cast, contingencies, etc…) to each unit in a region, and they then carry out the battle automatically whenever they attack or are attacked. In addition, if you want any information to strategize on, you must scout out a region, organize an attack, and hope your intelligence is still good by the time it launches.
 * To get reliable intel is almost impossible without special spies that not every nation has access to or spells that costs valuable magic gems. Some scrying spells even risk your casting mage's mind.
 * The non-responsiveness problem is heavily averted in Warcraft III: not only are units able to acquire and attack on their own, the "autocast" feature means that they will use certain abilities whenever appropriate. For instance, a Priest left alone with a group of units would automatically heal any injuries. (This tech actually debuted in Starcraft with the Terran Medic, but wasn't widely used until WC3.)
 * The autoretaliation debuted in Warcraft II, if not the first; it also had the feature of an attacking unit revealing itself through the Fog of War.
 * Also, Blizzard RTS games were some of the first to implement a "Hold Position" order, where a unit would stay in one spot and engage anything that came into range, but would not leave its position of attacked from range. This was useful if you were massing forces for an attack and didn't want them getting pulled into battle prematurely by enemy units trying to kite them into an ambush.
 * In Centurion Defender of Rome, during battles you can only change the orders of the units who are within the leader's range of voice (and each leader has a different one.)
 * In the original Dune PC game, initially you have to travel places to check on progress or issue orders, but as Paul's psychic ability increases you can contact outposts from further away, only becoming limitless after a forced plot evolution.
 * In Sword of the Stars Humans and Zuul cannot relay orders to fleets in nodespace because the signals wouldn't be able to catch up to the ships in time. The Tarka need to research a specific technology before they are able to communicate with ships in transit. Inside of battles, ships often take time to respond to orders (though this is more due to physical concerns rather than delays in communication) and can be set to a number of stances that limit or increase their personal initiative.
 * Interestingly played with in Achron: giving units commands in the past or the future costs chronoenergy. This leads to an interesting game mechanic of assigning units commanders to create chains of command: you can minimize chronoenergy expenditure by just giving orders to commanders and letting them communicate your orders to their underlings.
 * Averted hard in Gratuitous Space Battles. You can design the ships, giving them whatever weapons, engines, etc. you think will be most useful or economical, you choose what ships to deploy to the battlefield, their starting formations, and can even give each ship some general orders like "Stick Together" or "Stay near this ship" or "Retreat when badly damaged" or whatever. Once you hit the start button though, you can either sit back to watch the show, or go out for a beer, as either method will have equal impact on the battle. It can be maddening to watch some of your ships do something mind-bogglingly retarded and have no ability at all to tell them about it.
 * In a non-videogame example, this is mentioned in Tom Clancy's novel "The Bear and The Dragon" and explains why treating actual people in this manner is a bad idea.
 * Optional in Fields of Glory, a simulation of the climactic battles of Napoleon's Hundred Days campaign. Depending on the difficulty setting, units would take a certain amount of time to respond to your orders, which was explicitly justified on the basis that it would take time for orders to be relayed from the commander to the unit.
 * Slytherine's strategy game Spartan designed their mechanics with this specifically in mind. At the start of the battle it's possible to form your troops up and give them general strategy decisions (advance deep behind the enemy and flank) but once the battle is started it becomes impossible to relay orders more complex than retreat and rally.
 * Majesty averts this by not giving you direct control of "your" units at all - the best you can do is to post bounties for exploring locations or killing monsters, and whether the various heroes respond to them will depend on their own stats, morale, character type, and whether the bounty is sufficient to cover the effort involved. On the other hand, they'll respond to bounties as soon as you post them, so it's still in effect to some degree.
 * In Powermonger orders to your other captains are sent by carrier pigeon. If you like you can scroll the map and watch the bird fly to its destination.
 * In the American Civil War tactical game Take Command, messengers on horse are sent out from the field headquarters to the battallions with the planned orders, and these messengers can be killed by the enemy.

Justifications

 * Justified in Starcraft for the Zerg and Protoss factions because it is explicitly a psychic link.
 * Also might be justified for the Terran faction - the lower units like marines, firebats etc., who are brainwashed, drugged soldiers in power armors. The higher units like armors, fliers etc. are experienced and ranked. Actually, whenever you select multiple units, one of them (the one with the highest rank) is selected as a "command unit", which communicates with you. And it is in the future with rather few units (max 12 units get commands at the same time) - radio is quite fine for that, especially when you consider that there is hardly any cover and that taking cover with a ton heavy walking behemoth is not all that easy or practical for most cases. And for heroes... do we even have to go there? :)
 * The Terrans use Adjutants to control their troops, so it's not infeasible that, in fact, the commander's interface literally looks like an RTS and the AI relays orders. For example, when the commander "selects" a marine and then "selects" an enemy to attack, what actually happens is that the Adjutant translates it into orders communicated through the marine's Power Armor, via either voice or even by highlighting said enemy on the helmet's HUD. The real question here is who controls the units through the "RTS" overview screen in Starcraft II when Raynor is a playable unit on the battlefield, because the player is explicitly Raynor himself rather than some Non-Entity General.
 * Usually, missions with Raynor as a playable unit involve him personally leading a small unit. He could just be literally ordering his squad around verbally.
 * Anything with massively advanced communications or Psychic Powers can be expected to use something like this.
 * Total Annihilation and its Spiritual Successor Supreme Commander justify this - the entire army is robotic, so nigh-instant communications and perfect discipline are to be expected. They also partially avert the 'standing around uselessly' part of the trope - they will fire back at any unit in range unless ordered not to.
 * In Ender's Game, the Formics were like this due to a Hive Mind, and humans compensated by learning how to create instant communication technology.
 * It also does filter through a chain of command, albeit brief, once he started training with his squad leaders. This is usually not a problem...
 * The EVA units of Command and Conquer are noted as simplifying command of troops in the field, helping to justify that part of this trope in Tiberian Dawn, and, by extension, Tiberian Sun.