Months of dire warnings and diplomacy by the Biden Administration were ineffective in deterring Russian President Vladimir Putin from ordering his forces into eastern Ukraine, a move that American and European officials fear will herald a full-scale invasion.

Putin recognized the independence of Donetsk and Luhansk, two breakaway regions in eastern Ukraine controlled by Moscow-backed separatists, on Monday, and then directed Russian troops to occupy the territory for “peacekeeping functions.” The decision effectively nullified the Minsk cease-fire agreement, which was signed in 2014 after Putin illegally annexed Crimea in southern Ukraine.

Putin claimed in a fiery hour-long national address that Ukraine was historically a part of Russia and “never had traditions of its own statehood.” He also claimed that the country was now ruled by a “puppet regime” controlled by the United States and Europe.

The world’s attention is now focused on President Joe Biden and how he responds to Russia’s aggression and a volatile situation that could result in Europe’s largest conflict since World War II. According to administration officials, more than 50,000 Ukrainians could be killed in the crossfire of an invasion that extends all the way into Ukraine’s capital.

Biden spoke with Ukrainian President Volodymyr Zelensky on Monday and “strongly condemned Putin’s decision,” promising to “respond swiftly and decisively, in lockstep with its Allies and partners, to further Russian aggression against Ukraine,” according to a White House statement. In a nationally televised speech, Zelensky urged Ukrainians to remain calm and urged allies to show their support.

Biden signed an executive order prohibiting any US investment or trade in the regions that Putin was attempting to seize, but it did not impose any direct sanctions on Russia or its leadership. Despite earlier administration statements of a “swift and firm response,” a senior administration official who spoke to reporters on the condition of anonymity said the White House was still determining next steps.

The invasion represents a critical standoff between the United States and Russia, two nations with the world’s largest nuclear arsenals. Putin has successfully dragged Biden into a frustrating series of escalations, complicating the United States’ response to Russia’s actions, distracting from other diplomatic priorities, and raising the political stakes for the American president. Biden has stated repeatedly that he has no intention of involving the United States’ armed forces in another foreign war. His approval ratings plummeted after a tumultuous withdrawal from Afghanistan last summer, which allowed the Taliban to seize control of the country, and this is Biden’s second major foreign policy test as President.

Putin’s declaration came after several days of phone calls and meetings with world leaders in an attempt to persuade the Russian president not to launch an invasion. Russian troops, tanks, and artillery surround Ukraine on all sides: to the north in Belarus, to the east in Russia, to the south in Crimea, and to the west in Moldova. In recent days, the forces have moved closer to the border in positions that U.S. officials have described as indicating an impending invasion.

Biden ordered a 1,000-man Army Stryker squadron to move from Germany to Romania, a member of the North Atlantic Treaty Organization (NATO), to join the 900 US troops already stationed there. Another 1,700 troops from the 82nd Airborne Division and 300 from the 18th Airborne Corps are on their way to Poland, another NATO ally concerned about the security of its eastern border.

The United States has a small number of troops in Ukraine, which is not a NATO member. Biden has stated unequivocally that he has no plans to send in additional forces. Over the last year, he has sent more than $600 million in security assistance to the Ukrainian government in the form of weapons, equipment, and materiel.

Putin has repeatedly stated that he has no plans to send troops into Ukraine. Instead, he has repeatedly demanded that US forces withdraw from Eastern Europe and that other former Soviet-bloc countries, such as Ukraine, be barred from joining NATO—concerns that the US has already dismissed out of hand.