"One of the key reasons Putin probably felt comfortable launching the invasion of Ukraine was the extent to which the West has been weakened and destabilized, and democracy undermined, and political divisions sown, in the five years since he attacked our election in 2016," said Garrett Graff, a historian and journalist with expertise in national security and Cold War issues.

Here are four things to remember about Trump, Ukraine and Putin.

A popular revolution in 2014 ousted the pro-Russia regime in Kyiv, which was led by President Viktor Yanukovych, and replaced it with a Western-leaning government. Russian troops soon invaded the Ukrainian territory of Crimea, initiating the armed conflict that escalated this year.

Within weeks, Trump praised Putin for how he handled the takeover of Crimea and predicted that "the rest of Ukraine will fall ... fairly quickly." Echoing Kremlin propaganda, Trump said in a TV interview that the Crimean people "would rather be with Russia," a position he also pushed in private. One of his 2016 campaign aides falsely claimed that "Russia did not seize Crimea."

"Trump said that Crimea is Russian, because people speak Russian," said Elena Petukhova of Molfar, a Kyiv-based business intelligence firm, who called it an "absolutely pro-Kremlin" view. "According to this logic, the entire territory of the United States should belong to Great Britain."

When Russian-backed separatists in eastern Ukraine shot down a commercial airliner in 2014, killing 298 people, Trump sowed doubt about Russia's involvement. He embraced Putin's denials, even after US and European officials publicly concluded that Russia was complicit.

Trump's campaign chairman Paul Manafort -- who had spent a decade advising Yanukovych in Ukraine -- collaborated in 2016 with a Russian spy on a secret plan for Trump to help Russia control eastern Ukraine, according to special counsel Robert Mueller's report. The proposal envisioned that Yanukovych would return to lead a Russian puppet state in eastern Ukraine.