I Shall Return

There is a moment where a hero, such as a soldier of an elite commando force, barely escapes a high security headquarters with the help of fellow prisoners who have risked everything to help him escape, and now have to be left behind.

However, the hero tells them just before he leaves something along the lines of, "Don't despair, because I'll be back for you all." After considerable struggle, the hero returns to his headquarters and has more adventures until he has convinced his superiors to attack the enemy's base.

So, the soldier's unit returns to the villain's hideout in force and suddenly the prisoners hear the explosions and realize the soldier has returned as promised, and he's brought an army to liberate them!

The trope name comes from General Douglas MacArthur's catch phrase after he fled the Philippines in 1941 when the Japanese invaded, vowing that he would come back in force to liberate the nation, which he did years later with a hearty, "I have returned!"

Film

Speed: At the airport, Jack manages to convince Payne to let him off the bus to arrange the ransom. The other passengers are obviously unenthusiastic at being left behind, but Jack answers "Don't worry, I won't go far." A few minutes later, the hostages look in amazement to see Jack, a cop with apparently no fear, in front of them being towed on a service cart to attempt to disarm the bomb while the bus is in motion.

Live Action TV

Star Trek: Deep Space Nine: In "Call to Arms", Sisko tells the people of DS9, "I will not rest until I stand with you again," before leaving to fight the Dominion.
At the end of the first episode of Blakes Seven, a guard on the deportation ship warns his prisoners to take a last look at Earth—it's the last time they'll ever see it. Blake replies, "No, I'm coming back," with a manner that suggests he's just going on holiday. (He does indeed return.)

Toys

Bionicle: at the end of the 2008 storyline, Mata Nui, whose body was stolen by Makuta and is now sealed in the Mask of Life, vows "I will return!"

Western Animation

G.I. Joe: The Mass Device: Duke made this promise to a woman who helped him escape and he came back with an assault force.
World's Greatest Superfriends: Superman was trapped in the "Universe of Evil" and just got some vital help from a scientist who asks him to come back one day and free her world. Superman responds, "You can count on it!" The episode ends with the Superfriends' preparation to invade.
After the failed invasion of the Fire Nation in Avatar: The Last Airbender, Aang gives a brief speech along these lines to the soldiers that are forced to surrender before escaping on Appa. Sokka furthermore promises his father that "we won’t be apart for too long this time."