A dangerous, tense situation exists in Eastern Europe between the independent country of Ukraine and Russia, a much larger neighbor on Ukraine's eastern and southern sides. Russian President Vladimir Putin has sent soldiers and missiles near the border, prompting efforts by President Biden and others to avert armed conflict. "I'm convinced he's made the decision ... [to start a] "catastrophic and needless war of choice," Biden said Friday in a televised address. "It is not too late to de-escalate and return to the negotiating table," the president added, referring to a meeting scheduled Thursday in Europe between Secretary of State Antony Blinken and Russia's foreign minister. "If Russia takes military action before that date, it will be clear that they have slammed the door shut on diplomacy." Ukraine's president, Volodymyr Zelensky, said Saturday that his country "has been serving as a reliable shield for eight years now, holding back one of the largest armies in the world. This is not a war in Ukraine, but a war in Europe."

With an estimated 160,000 to 190,000 troops on three sides of Ukraine, the Russian Army could quickly overwhelm the smaller country's less numerous military forces. The American president rules out sending U.S. troops to fight in Ukraine, but American-made weapons are already there in force and more will be on the way. How effective they would be in turning back a Russian invasion is another question. In addition, Biden has sent 3,000 troops to the region, including -- 1,700 American soldiers in Poland -- in case they're needed to help with evacuations, medical needs and intelligence. The situation is Biden’s biggest foreign policy crisis since America's messy pullout from Afghanistan last August.

Putin worries about a potential NATO invitation for Ukraine to join, an issue at the center of the current tension. The 30-member alliance already includes Latvia and Estonia, both on Russia's western boundary, plus nearby Lithuania and Poland. Putin wants a rollback of NATO's presence in Eastern Europe and a guarantee that Ukraine won’t join.

Ukraine became part of the former Soviet Union after World War II and gained independence in 1991 when that superpower dissolved. (Its formal name was the Union of Soviet Socialist Republics, or USSR.) Ukraine now is Europe's second-largest country by area, behind Russia. After the Soviet Union’s collapse 31 years ago, many Russian politicians viewed the divorce with Ukraine as mistake. The Black Sea fleet in Russia's navy was stationed there and Ukraine was critically important as an agricultural area. Russia was Ukraine's largest trading partner for a long time, though China now fills that role. Letting Ukraine fall into the Western orbit was seen by many as a major blow to Russia's international prestige.

President Biden says: "We believe that they will target Ukraine's capital, Kyiv, a city of 2.8 million innocent people." -- Address last Friday

U.S. diplomat says: "Russia probably has massed between 169,000-190,000 personnel in and near Ukraine as compared with about 100,000 on January 30." -- Michael Carpenter, ambassador to the Organization for Security and Cooperation in Europe

Historian says: "At stake in this crisis is the concept of the international rule of law. The obvious question is whether nations should control their own borders and governments, or whether larger countries can absorb others." – Heather Cox Richardson, Boston College