1999 Russian apartment bombings
In September 1999, a series of explosions hit four apartment blocks in the Russian cities of Buynaksk, Moscow, and Volgodonsk, killing more than 300, injuring more than 1,000, and spreading a wave of fear across the country. The bombings, together with the 1999 war of Dagestan, triggered the Second Chechen War.
Source: Wikipedia — 1999 Russian apartment bombings (CC BY-SA 4.0)