There are those who believe (as I do) that
Yellow Submarine is an allegory for World War II, or the Twentieth Century European wars in general. Here are some of the supposed congruences.
...it goes on and on. Admittedly, some of these derive
from each other instead of supporting each other; however, it still makes the movie more fun to watch.
The initial invasion of Pepperland equates to the Anschluss and blitzkrieg invasion of Poland. The Lord Mayor's denial of the invasion represents the air of denial and appeasement that prevailed at the time, and the dispatch of Young Fred to get help signified the Dunkirk debacle.
If we continue on in this vein, the Beatles agreeing to help would represent the people of the Allies agreeing to fight the threat and pitching in as democracies (mostly).
I don't know to what degree this makes sense and to what degree it's self-constructed and self-supported, but it does make for fun during viewings.