Germany has crossed a red line with Russia by sending arms to Ukraine, Moscow’s ambassador in Berlin said on Monday. The decision undermined decades of reconciliation since the end of World War II and the Nazi invasion of the Soviet Union, the diplomat added.

“The very fact that the Ukrainian regime is being supplied with German-made lethal weapons, which are used not only against Russian military service members, but also the civilian population of Donbass, crosses the red line,” Ambassador Sergey Nechaev said in an interview with Izvestia newspaper.

He added that Berlin should have known better, “considering the moral and historic responsibility that Germany has before our people for the Nazi crimes.”

“They have crossed the Rubicon,” Nechaev stated, using an idiom for passing the point of no return.

Berlin discarded its longstanding policy of not sending weapons into zones of armed conflict to join the US and other NATO allies in providing weapons to Ukraine. The German government says it has a moral responsibility to back Kiev so it can defend itself against Russia. Germany also joined an effort by the EU to decouple the economies of member states from Russia’s. German businesses have been relying on cheap Russian natural gas for five decades, since before the Soviet Union collapsed. The German government “has unilaterally acted to destroy bilateral relations [with Russia] that were unique in scale and depth and had been built over decades,” the Russian ambassador noted. “In essence, the post-war reconciliation of our nations and peoples is being eroded,” Nechaev said.