Japan and America fought in WWII.
We battle against this gigantic monster called kanji every day so we can get good at Japanese.
Kendo is normally a one-on-one martial art, but 5 years ago Koichi had to fight against two guys at the same time.
This is the verb version of the vocab word 戦い, which I'm hoping you learned earlier. The verb version of course means to fight or to battle.
The reading comes from 戦い a word you've hopefully seen by now. Associate these words together in your mind and you'll be able to read them so long as you know the other one.