Background: War (originally called Eric Burdon and War) is an American funk band from Long Beach, California, known for several hit songs (including "Spill the Wine", "The World Is a Ghetto", "The Cisco Kid", "Why Can't We Be Friends?", "Low Rider", and "Summer"). Formed in 1969, War is a musical crossover band which fuses elements of rock, funk, jazz, Latin, rhythm and blues, and reggae. Their album The World Is a Ghetto was the best-selling album of 1973. The band also transcended racial and cultural barriers with a multi-ethnic line-up.
Context: Eric Burdon and War toured extensively across Europe and the United States. A reviewer from New Musical Express called War "the best live band I ever saw" after their first UK gig in London's Hyde Park. Their show at Ronnie Scott's Club in London on September 18, 1970 is historically notable for being the very last public performance for Jimi Hendrix, who joined them onstage for the last 35 minutes of Burdon's & War's 2nd set; a day later he was dead. A second Eric Burdon and War album, a two-disc set titled The Black-Man's Burdon was released in 1970, before Burdon left the band in the middle of its European tour. They finished the tour without him and returned to record their first album as War. War (1971) met with only modest success, but later that year, the band released All Day Music which included the singles "All Day Music" and "Slippin' into Darkness". The latter single sold over one million copies, and was awarded a gold disc by the R.I.A.A. in June 1972. In 1972 they released The World Is a Ghetto which was even more successful. Its second single, "The Cisco Kid" shipped gold, and the album attained the number two spot on Billboard Hot 100 chart, and was Billboard magazine's Album of the Year as the best-selling album of 1973. The next album, Deliver the Word (1973) contained the hits "Gypsy Man" and a studio version of "Me and Baby Brother" (previously issued as a live recording), which peaked at #8 and #15 on the Billboard chart. The album went on to sell nearly two million copies. The next album, Why Can't We Be Friends? was released in 1975. It included "Low Rider" and the title track, which were among the band's biggest hits. In 1976, War released a greatest hits record which contained one new song "Summer", which, as a single, went gold and peaked at number 7 on the Billboard chart. Also released that year were Love is All Around by Eric Burdon and War, containing mostly unreleased recordings from 1969 and 1970, and Platinum Jazz, a one-off album for jazz label Blue Note. The latter double album had cover art to match the greatest hits album, and was half new material and half compilation, focusing on (but not restricted to) instrumental music. The group continued to attain success with their next album, Galaxy (1977) whose title single was inspired by Star Wars. War's next project was a soundtrack album for the movie Youngblood in 1978.
Question: What was their most popular album?. Whats the answer?
"Slippin' into Darkness". The latter single sold over one million copies, and was awarded a gold disc by the R.I.A.A. in June 1972.