Duck Soup

The last man nearly ruined this place, he didn't know what to do with it.
If you think the country's bad off now, just wait till I get through with it!

Duck Soup is a 1933 movie, starring the Marx Brothers. The movie was added to the National Film Registry in 1990.

Groucho Marx plays Rufus T. Firefly. Mrs. Teasdale (Margaret Dumont's character) has appointed him ruler of Freedonia. One of his first acts in office, after promising to be a bad president and insulting her, is to offend the ambassador of Freedonia's neighbor, Sylvania.

Chico Marx and Harpo Marx are two Sylvanian spies. They are about as competent as you can expect from characters played by Chico and Harpo.

Twice, Firefly considers making peace with Sylvania. Twice, he chooses to offend the ambassador further. Finally, Sylvania declares war.

Hilarity Ensues throughout all these proceedings.

Not to be confused with the 1927 Laurel and Hardy short with the same name, nor with oritang, the Korean spicy duck soup.

Tropes used in Duck Soup include:

Anti-Hero: Rufus T. Firefly, as summed up with the song lyrics included at the top of the page.
Awesome McCoolname: Rufus T. Firefly.
Crowd Song: "To war, to war, to war we gotta go!" It eventually devolves into a minstrel show (no Blackface, though).
Cute Mute: Harpo, although this is one of the films that does address the fact that he is mute by choice and used to talk (Chicolini can't understand why he won't talk to him).
Distracted by the Sexy: Firefly sends Pinky out to warn the country that the Sylvanians are coming, which he does, in a Paul Revere-esque manner, until... he sees a woman about to take a bath. And yet he prefers to share her bed with his horse.
Fat Girl: As Firefly says of Mrs. Teasdale:

"I can see you now in the kitchen, bending over a hot stove. But I can't see the stove!"

Fiction 500: Mrs. Teasdale is rich enough to solve a nation's debt, even a small one, and declare who is to lead it.
Grande Dame: Mrs. Teasdale.
Hilarity Ensues: It's the Marx Brothers. You could make them fill out taxes and still get this.

Taxes? That's where my uncle is from! Dollars, Taxes!

Hurricane of Puns: Chicolini's trial. The above quote is just the tip of the iceberg.
I Know You Know I Know
Implausible Deniability: "Who you gonna believe, me or you own eyes?"
Incessant Chorus: The Freedonian national anthem.
Insult to Rocks: After calling the Sylvanian ambassador a baboon, Firefly apologizes, saying "It isn't fair to the rest of the baboons."
I Take Offense to That Last One
"Join the Army," They Said: And see the Navy.
Mirror Routine
One-Liner
Premature Aggravation: Used to provide the page quote.
Pretty in Mink: Mrs. Teasdale wears a couple, given she's rich enough to bail out a bankrupt country.
Produce Pelting: Firefly and company do this to Trentino at the end of the movie. Then they do this to Mrs. Teasdale when she insists on singing the Freedonian national anthem again.
Rapid-Fire Comedy
Right on the Tick: Firefly is supposed to turn up when the clock strikes 10. Subverted for laughs: he sneaks in from the back.
Rule of Funny: The answer to any of the MANY gaps in logic in the film. It works because just about every gap results in a Crowning Moment of Funny.
Running Gag:
- Harpo pulls up in a motorcycle, Groucho gets into the sidecar, Harpo drives off. Invariably, Groucho is left behind.

Groucho: This is the third trip I've taken today, and I haven't gone anywhere.

- Later subverted as Groucho gets clever and insists on riding on the motorcycle portion. Cue Harpo driving off in the sidecar.

Groucho: Ah, this is the only way to travel.

- Groucho's changing uniforms during the actual war sequence.
- Harpo's half-kleptomania, where he steals half-items by snipping them off with scissors.
Senseless Violins: Groucho of all people in the battle sequence. Naturally, he's shooting in the wrong direction.
Silence Is Golden: The Mirror Routine is performed entirely without sound.
Slapstick
Stock Footage: Amusingly deployed when Firefly summons for help during the battle sequence.
Stop Helping Me!:
- When Freedonia is being bombed, Groucho gets a huge white vase jammed down over his head. Harpo's idea of helping is to draw Groucho's face on the outside of the vase.
- Remember that people still sometimes used chamber pots and Groucho's line becomes Fridge Nausea:

"Last time this happened to me I was crawling under a bed!"

That Was Objectionable: The treason trial.
This Means War: Not the trope creator, but possibly the Trope Namer. Firefly declares war after insulting himself and blaming it on the other country's ambassador. And then there's a big musical number about it.
You Fool!: The Sylvanian ambassador uses a few lines in response to the insults from Groucho. The one that sets off the slap is "You upstart".