DIE OPPRESSORS!
Explanation
The Joke
A group of revolutionaries triumphantly declare: "We have overthrown the oppressors! Let our flag be the color of blood! And let its emblem be a sword of nine blades!" The next panel reveals that the flag they have just described is... the Canadian flag. The maple leaf is reimagined as a "sword of nine blades" and the red and white of the Canadian flag matches the "color of blood" description.
The Humor
The humor comes from the stark contrast between the violent, revolutionary rhetoric and the resulting flag, which belongs to Canada -- a nation internationally stereotyped as being excessively polite, peaceful, and non-threatening. The comic reframes the familiar Canadian maple leaf flag through the lens of aggressive revolutionary iconography, making the audience realize that if you describe the Canadian flag in the most dramatic terms possible, it actually sounds terrifyingly militant. The joke also plays on the fact that many national flags do have origins in revolutionary violence, and Canada's flag, despite its benign associations, features design elements (red for blood/sacrifice, a pointed leaf that could be read as a weapon) that could theoretically be given a fearsome interpretation.
References
The Canadian flag (the Maple Leaf) was adopted in 1965 and features a red maple leaf on a white background flanked by red bars. The red is sometimes said to represent sacrifice and the blood shed in war, which makes the comic's reinterpretation historically adjacent to actual flag symbolism.