How did the U.S. become so involved with this former Soviet republic? Here's everything you need to know:

How did modern Ukraine emerge?
Ukraine's history is intertwined with Russia's. Both cultures descend from the medieval Slavic empire Kievan Rus, founded by Vikings in the 9th century with a capital where Kyiv stands today. After the empire was overrun by Mongol invaders in the 13th century, the Rus leadership moved northeastward, to a small trading outpost called Moscow. The lands around Kyiv were carved up by competing powers, who prized the fertile plains and rich, dark soil that later earned Ukraine the nickname "the breadbasket of Europe." Poland and Lithuania dominated the country for hundreds of years, but by the end of the 18th century, the Russian Empire had taken the eastern lands, while the western were controlled by the Austro-Hungarian Empire — a division that resonates to this day. Both empires collapsed at the end of World War I, and in 1917 a Ukrainian state was declared, but it was quickly swallowed by the communist Soviet Union. It was only in 1991, after the Soviet Union dissolved, that the modern borders of Ukraine were established. Russia has never really accepted Ukraine's independence, and Ukraine has been careening between pro-Western and pro-Russian leaders ever since.

Why do Russians still see it as theirs?
The cultural and Orthodox religious ties between the two peoples go back centuries. But perhaps more importantly, Russia has no natural mountain or river borders to protect its western front, and it has long seen Ukraine as a vital strategic buffer. "Russia without Ukraine is a country," explains Daniel Drezner, an international politics professor at Tufts University. "Russia with Ukraine is an empire." Russian President Vladimir Putin wants at all costs to keep Ukraine out of the European Union and NATO, and he has used Ukraine's dependence on Russian natural gas as a weapon — cutting off supplies in 2006 as Ukrainians froze in the winter cold. Ukraine is bitterly divided between Ukrainian speakers in the west, who mostly want to align their country with the West, and Russian speakers in the east.

Why did the U.S. get involved?
Since 1991, the U.S. has sought to guarantee Ukraine's independence and security, seeing it as a key bridge between East and West. When the USSR dissolved, Ukraine inherited the third-largest nuclear arsenal in the world, with some 1,900 strategic nuclear weapons pointed at the United States. Keen to avoid a proliferation of nuclear powers, the U.S., U.K., and Russia in 1994 signed the Budapest Memorandum, which transferred the weapons to Russia in exchange for a commitment by Washington, Moscow, and London to "respect the independence and sovereignty and existing borders of Ukraine" and to "refrain from the threat or use of force" against it. Russia continued meddling in Ukrainian politics, though, and overtly broke the treaty in 2014.

What did Russia do?
Ukraine's 2004 election was a showdown between pro-Western Viktor Yushchenko and pro-Russian Viktor Yanukovych. During the campaign, Yushchenko was disfigured by dioxin poisoning widely attributed to Russia, and after the vote was rigged for Yanukovych, Ukrainians rose up in the Orange Revolution, which culminated in a revote and a victory for Yushchenko. In the 2010 election, though, Yanukovych came roaring back to win the presidency with the help of both Russia and American political consultant Paul Manafort. In 2013, Moscow forced Yanukovych to cancel the country's bid to move closer to the EU, leading to an uprising that forced Yanukovych out. In the tumult that ensued, Russia invaded and annexed Crimea and later sent troops into Ukraine's eastern regions to start a separatist war. The Obama administration punished Russia with sanctions, but chose not to send weapons to Ukraine out of fear of triggering a larger war.

How is the war going?
Armed by Russia and supported by Russian paramilitaries, two ­Russian-­speaking Ukrain­ian regions collectively called the Donbass declared independence in 2014 and have been fighting with Ukraine ever since. An estimated 13,000 people have been killed and some 1.5 million displaced. In 2018, then–­national security adviser H.R. McMaster and other aides persuaded a reluctant Pres­i­dent Trump to send $400 million in military aid to Kyiv, including counter-artillery radar systems and Javelin anti-tank missiles. Without that aid, Mariya Omelicheva of the Pen­ta­gon's Nation­al De­fense Uni­ver­si­ty told The Atlan­tic, the 300-mile-long front line in the eastern Don­bass region "would have been moved further west into Ukraine, and ­Russia-backed rebels would have controlled more Ukrain­ian territory." That's why the possibility of U.S. aid being held up — the subject of the impeachment ­inquiry — is so important to Ukraine. Ukraine's new president, Volody­myr Zelen­sky, finds himself caught in a struggle between Democrats and Trump — who has reportedly been echoing Vladimir Putin's claim that Ukraine is corrupt and "not a real country."

Americans cash in
As Ukraine emerged from communism and struggled to build a rule-of-law economy, politically connected Amer­i­cans saw ways to make money. Paul Mana­fort, President Trump's former campaign manager, earned at least $13 million — and possibly much more — ­working for pro-­Russian Ukrainian oligarchs. His consultancy for a pro-Russian party that tried to undermine the U.S. alliance with Ukraine was crucial to the triumph of Yanu­ko­vych in the 2010 election. Mana­fort is now serving 7½ years in prison for tax fraud over those earnings. Joe Biden's son Hunter joined the board of Buris­ma, a Ukrain­ian gas company, in 2014 at a reported salary of tens of thousands of dollars a month, although there is no indication that either Biden broke any laws. Rudy Giu­li­ani, meanwhile, the president's personal lawyer, had a 2017 ­security-­consulting contract with the Ukrain­ian city of Kharkiv, and in 2018 he began working with two Soviet-born Amer­i­can executives, Lev Parnas and Igor Fruman, Repub­li­can donors who wanted to sell gas to Ukraine. Federal prosecutors have indicted Par­nas and Fru­man for illegal campaign contributions and are now reportedly investigating whether Giu­li­ani "stood to profit personally from a Ukrain­ian natural-gas business" pushed by those two men.