After months of military and political reinforcement of brinkmanship, US President Joe Biden on Tuesday called Russia’s moves in Ukraine “the beginning of a Russian invasion.”

This is what you should know:

What did Russia do? Russian President Vladimir Putin on Monday ordered troops into Donetsk and Luhansk, two pro-Moscow separatist-held regions in eastern Ukraine, in what the Kremlin called a “peacekeeping” mission. This step came just hours after he signed decrees recognizing the independence of the regions.

Several US and Western officials have warned that this could amount to a shooting spree in a larger military operation against Ukraine. More than 150,000 Russian soldiers are now besieging Ukraine on three sides, according to estimates by US and Ukrainian intelligence officials.

How is Ukraine responding? On Tuesday, Ukrainian President Volodymyr Zelensky said he was still practicing diplomacy as a way out of the crisis. He said reservists would be called up for military training, but there would be no general mobilization of the armed forces.

“We want peace and quiet, but if we calm down today, we will disappear tomorrow,” Zelensky said.

How did the world react? Russia’s actions were strongly condemned by many countries, with Western leaders imposing new sanctions on Tuesday and cutting a major oil pipeline with Russia.

Biden announced that the United States would impose sanctions on Russian financial institutions and oligarchs. The European Union also imposed sanctions on 351 Russian lawmakers who voted to recognize the breakaway regions, and the United Kingdom announced sanctions on five Russian banks and three Russian oligarchs.

Also Tuesday, Germany said it was halting certification of an $11 billion gas pipeline connecting Russia directly to Germany. Nord Stream 2 was completed in September but has yet to receive the final green light from German regulators. Without it, natural gas could not flow through the Baltic Sea pipeline from Russia to Germany.

Japan and Australia on Wednesday joined the list of countries imposing sanctions on Russia, Donetsk and Luhansk.