From the collapse of the USSR:

The USSR dissolved in 1991 under pressure from a new alliance of Russia, Belarus, and Ukraine, joined by most of the other Soviet republics. Quickly, well-connected businessmen in those former republics began to amass wealth and power. At the same time, the fall of the Soviet Union prompted lawmakers in the U.S. to champion the free enterprise they were convinced had sunk the Soviets. They deregulated the U.S. financial industries just as rising oligarchs in Eastern Europe were eager to launder illicit money.
In 1999, Poland, Hungary, and the Czech Republic, all former satellites of the USSR, joined NATO over the protests of Russia, which was falling under the control of oligarchs who opposed western democracy. More countries near Russia joined NATO in the 2000s.
Russia set out to keep control of Ukraine. In 2004, it appeared to have installed a Russian-backed politician, Viktor Yanukovych, as president of Ukraine, but Yanukovych was rumored to have ties to organized crime, and the election was so full of fraud—including the poisoning of a key rival who wanted to break ties with Russia and align Ukraine with Europe—that the government voided the election and called for a do-over.
In 2004, Yanukovych began to work with U.S. political consultant Paul Manafort, who was known for managing unsavory characters, and in 2010, Yanukovych finally won the presidency on a platform of rejecting NATO. Immediately, Yanukovych turned Ukraine toward Russia. But in 2014, after months of popular protests, Ukrainians ousted Yanukovych from power in what is known as the Revolution of Dignity. He fled to Russia.
Shortly after Yanukovych’s ouster, Russia invaded Ukraine’s Crimea and annexed it, prompting the United States and the European Union to impose economic sanctions on Russia itself and also on specific Russian businesses and oligarchs, prohibiting them from doing business in U.S. territories. Since Russians had been using U.S. financial instruments to manage their illicit money, these sanctions froze the assets of key Russian oligarchs.