Complex sentence: In 1927, he became the fourth World Chess Champion by defeating José Raúl Capablanca, widely considered invincible, in what would stand as the longest chess championship match held until 1985.
Split into simpler sentences: In 1927, he became the fourth World Chess Champion by defeating José Raúl Capablanca, who was widely considered invincible. That would stand as the longest chess championship match held until 1985.