Ukraine was a cornerstone of the Soviet Union until it voted overwhelmingly for independence in a democratic referendum in 1991, a milestone that turned out to be a death knell for Russia.
NATO pushed eastward, bringing into the fold most of the Eastern European nations. In 2004, NATO added the former Soviet Baltic republics Estonia, Latvia and Lithuania.
Four years later, it declared its intention to offer membership to Ukraine in the distant future crossing a red line for Russia. Putin indicated he sees NATO's expansion as an existential threat.
In interviews and speeches, Vladimir Putin has emphasized his view that Ukraine is part of Russia, culturally, linguistically and politically and Russian speaking people in Ukraine agree to it.
In early 2014, mass protests in the capital Kyiv known as Euromaidan forced out a Russia-friendly president after he refused to sign an EU association agreement.
Russia responded by annexing the Ukrainian peninsula of Crimea and fomenting a separatist rebellion in Ukraine's east, which seized control of part of the Donbas region.
Despite a ceasefire agreement in 2015, the two sides have not seen a stable peace, and the front line has barely moved since. Nearly 14,000 people have died in the conflict.
In the eight years since, Moscow has been accused of engaging in hybrid warfare against Ukraine, using cyberattacks, economic pressure and propaganda to whip up discord.
Those tactics have escalated in recent months, and in early February the State Department claimed Putin was preparing a false-flag operation to create "a pretext for an invasion."