A long-simmering power struggle in Malaysia boiled over on Monday, with Prime Minister Mahathir Mohamad kicking off a leadership race with his main coalition partner that could finally determine his successor.

Mahathir abruptly submitted his resignation to the king on Monday and his party exited the ruling coalition. The monarch accepted it, appointed Mahathir as interim prime minister until a new leader emerges and dismissed the cabinet.

While the outcome of the power struggle is unknown, it's another stunning twist in the decadeslong rivalry between Mahathir and Anwar Ibrahim. The distrust between them dates back to the 1990s, when Anwar was ousted from Mahathir's cabinet and arrested for sodomy.