Putin's power play: Russia's president threatens neighboring Ukraine-and alarms the world.

• Author: Smith, Patricia
• Position: INTERNATIONAL - Vladimir Putin

In February, the world's attention was focused on Russia as it successfully hosted the Winter Olympics in Sochi. A month later, all eyes are on Russia for a more troublesome reason: its military intervention in neighboring Ukraine. President Vladimir Putin seems to have squandered all the good press Russia received from the Games-and the $50 billion they cost-by sending troops into Ukraine. It's a move that calls to mind the Soviet Union's aggression during the Cold War. Putin acted following the ouster of Ukraine's president by pro-Western protesters upset at his rejection of a trade agreement with Europe. Here's what you need to know as these events continue to play out.

Why did Putin send in troops?

Putin says the upheaval in Ukraine is a threat to Russia and Russian citizens in Ukraine, and he needs to protect them.

But the larger reason is the long history linking Ukraine and Russia. Until the Soviet Union collapsed in 1991, Ukraine was one of 15 separate republics (including Russia) that made up the Soviet Union. Many ethnic Russians still live in Ukraine--especially in the eastern part of the country, including the Crimean peninsula. They've traditionally had closer ties to Moscow than to the rest of Europe, and they've welcomed the Russian troops.

Other Ukrainians fear that Russia will tighten its grip and prevent pro-Western policies from taking hold in Ukraine. "They are a powerful country," a 20-year-old in Kiev told CBS News. "They can do bad things with us."

Is Putin trying to bring back the Soviet Union?

When the Soviet Union collapsed, its republics became independent nations. Russia was left with 23 percent less land and half the population of the Soviet Union. Many Eastern European countries, which had been satellite nations under Soviet control, became Western-leaning democracies.

Putin has described the collapse of the Soviet Union, which left the U.S. as the sole superpower, as "the greatest geopolitical catastrophe" of the 20th century.

So it's no surprise that Russia has repeatedly intervened in the affairs of its former republics. In 2008, it sent troops into Georgia in response to a conflict over two small regions with strong Russian ties. The international community was outraged but did little. At the time, Georgia's president said Russia was "bent on restoring a neocolonial form of control over the entire space once governed by Moscow."

How has the U.S. responded?

The day before Russian troops intervened in...