Garry Kimovich Kasparov (Russian: Garri Kimovich Kasparov, Russian pronunciation: ['garjI 'kjim@vjItc ka'spar@f]; born Garik Kimovich Weinstein, 13 April 1963) is a Russian, and formerly Soviet, chess grandmaster, former world chess champion, writer, and political activist, who many consider to be the greatest chess player of all time. From 1986 until his retirement in 2005, Kasparov was ranked world No. 1 for 225 out of 228 months. His peak rating of 2851, achieved in 1999, was the highest recorded until being surpassed by Magnus Carlsen in 2013. Kasparov also holds records for consecutive professional tournament victories (15) and Chess Oscars (11).

As a teenager, Kasparov tied for first place in the USSR Chess Championship in 1981-82. His first win in a superclass-level international tournament was scored at Bugojno, Bosnia and Herzegovina in 1982. He earned a place in the 1982 Moscow Interzonal tournament, which he won, to qualify for the Candidates Tournament. At age 19, he was the youngest Candidate since Bobby Fischer, who was 15 when he qualified in 1958. At this stage, he was already the No. 2-rated player in the world, trailing only World Chess Champion Anatoly Karpov on the January 1983 list.  Kasparov's first (quarter-final) Candidates match was against Alexander Beliavsky, whom he defeated 6-3 (four wins, one loss). Politics threatened Kasparov's semi-final against Viktor Korchnoi, which was scheduled to be played in Pasadena, California. Korchnoi had defected from the Soviet Union in 1976, and was at that time the strongest active non-Soviet player. Various political maneuvers prevented Kasparov from playing Korchnoi, and Kasparov forfeited the match. This was resolved by Korchnoi allowing the match to be replayed in London, along with the previously scheduled match between Vasily Smyslov and Zoltan Ribli. The Kasparov-Korchnoi match was put together on short notice by Raymond Keene. Kasparov lost the first game but won the match 7-4 (four wins, one loss).  In January 1984, Kasparov became the No. 1 ranked player in the world, with a FIDE rating of 2710. He became the youngest ever world No. 1, a record that lasted 12 years until being broken by Vladimir Kramnik in January 1996; the record is currently held by Magnus Carlsen.  Later in 1984, he won the Candidates' final 8 1/2 -4 1/2  (four wins, no losses) against the resurgent former world champion Vasily Smyslov, at Vilnius, thus qualifying to play Anatoly Karpov for the World Championship. That year he joined the Communist Party of the Soviet Union (CPSU), as a member of which he was elected to the Central Committee of Komsomol in 1987.

Answer the following question by taking a quote from the article: How did politics prevent him from playing?
Korchnoi had defected from the Soviet Union in 1976, and was at that time the strongest active non-Soviet player.