“Joins late and does all the work”

You forgot the rest of it:
1) organized r*pe of women from captured countries.
2) Killing people who surrendered.
3) various other war crimes.

And cannibalism!

First off, is being attacked not a feasible reason to attack back? Second, Britain was getting slowly strangled.

The joke was that Japan bombed Pearl Harbour and destroyed 6 of USA’s ships, and the war ended with USA basically radiating two Japanese cities, and it sounds funny/excessive back to back. For the Britain one it just sounds better without the details.