Well, most is an exaggeration, I think. Some, yes. But he started on them before WWII began (way before, if you include The Hobbit and ideas for the Silm), and continued them well after. LOTR Trilogy wasn't published until mid-50s.
Also, he wasn't a war correspondent, he worked Intelligence for the British Army, IIRC.
EDIT: correction -- wasn't the Army, was the Foreign Office (Brit equivalent to US State Dept). Just looked it up.