From Wikipedia:

During the sessions for "Sunday Bloody Sunday", Lillywhite encouraged drummer Larry Mullen, Jr. to use a click track, but Mullen was firmly against the idea. A chance meeting with Andy Newmark (of Sly & the Family Stone) – a drummer who used a click track religiously – changed Mullen's mind. Mullen used the click track to stay in time for other songs on the album. Mullen said of the album in a 1983 interview, "I think the drumming has always been pretty simple, I don't think it needs to be flashy. For War I use a click track, something I haven't used before, it's a way of keeping time in my headphones. When I listened to the music in time with the click track I knew I had to bring it down to the real basics. Hopefully for the next LP it will be more complicated, I'll move on. I think of it as a musical progression for myself because I learned a lot recording this album, just about my own style and that's what I wanted to do. I think there is a definite style on War where there isn't on the previous albums."