Let us consider how ludicrous the setup for this short is. During World War 2, Donald Duck sets up his own makeshift listening post on the California coast staffed by his three juvenile nephews in order to guard against impending Japanese invasion. Their weaponry consists of several shotguns tied together as an anti-aircraft gun that would only have the effective range of 100 yards or so, a few wooden swords (?!), and a circus cannon you'd see more fit to launch trapeze artists than any serviceable ordinance. Was Donald acting under orders by the Army or did he set this all up (incompetently) on his own accord out of his personal brand of patriotic duty? Watch and find out (or not)!
Really it's surreal but presented in such a matter-of-fact way that kid audiences won't really care and just see it as a loose setup for gags of Donald and his nephews picking on each other. The two biggest laughs come from the model airplane / gingerbread man invasion prank (complete with Donald's terrified reaction), along with the ultimate showdown between his cannon and hapless bee on maximum volume. It's Donald's attitude, enthusiasm, and proclivity to anger that makes this all works so well.
There were a few other Donald Duck wartime propaganda cartoons which had bigger and better laughs to them, but this one stands out with the setting and ridiculously over the top climax. The 1940's easily were Disney's high point in terms of excellent animation and humor that worked for adult and child audiences alike, so it's no surprise how well most of their wartime propaganda cartoons worked. The shock for me these days comes from thinking about how (or if) these cartoons really did much to instill a sense of patriotism in audiences that the government wanted at the time, as they largely depicted the U. S. military (or homegrown militia as presented here) as foolishly inept. I guess nobody was looking into the greater subtext at the time, or maybe they were designed more to subtly poke fun at the propaganda of the day rather than perpetuate it? We can only speculate these days as the answers likely have long gone lost in time.