BELGRADE, Serbia (AP) — Chinese leader Xi Jinping’s visit to European ally Serbia on Tuesday falls on a symbolic date: the 25th anniversary of the bombing of the Chinese Embassy in Belgrade during NATO’s air war over Kosovo.

U.S. jets dropped five bombs on the Chinese Embassy compound in the Serbian capital on May 7, 1999, setting it ablaze and killing three Chinese nationals. Twenty other people were injured in the incident, which has burdened relations between the two powers ever since.

Xi referred to the bombing in an op-ed published in Serbia’s Politika newspaper on Tuesday, saying that “we must not forget that 25 years ago today, NATO brazenly bombed the Chinese Embassy in Yugoslavia,” according to translations carried by Chinese state media.

“The Chinese people value peace but will never allow historical tragedies to happen again,” Xi added.