This song is about man's desire to kill and destroy. As stated in Black Sabbath guitarist Tony Iommi's memoir Iron Man: My Journey through Heaven and Hell with Black Sabbath, they got the idea from war stories they heard when they did a show at an American Air Force base during a tour of Europe. They wrote the song when they were in a grim, deserted place in Zurich playing for a small sum of money to an even smaller audience.

"War Pigs" was inspired by the Vietnam War, which was raging in 1970 when Black Sabbath included the song on their second album, Paranoid. Sabbath bass player Geezer Butler, who was also their lyricist, recalled to Mojo in 2017: "Britain was on the verge of being brought into it, there was protests in the street, all kinds of anti-Vietnam things going on. War is the real Satanism. Politicians are the real Satanists. That's what I was trying to say." Another song on the album, "Hand Of Doom," also drew inspiration from the Vietnam War. That one is a look at soldiers who survive the war but can't live through the ensuing trauma when they return home.

Speaking with Songfacts in 2024, Geezer Butler said "War Pigs" was the Black Sabbath song that remained most relevant, "because it never goes away."