The revolutionary violence that swept Kyiv’s Maidan Square on the night of February 21, 2014 unleashed the forces of Ukrainian nationalism and, ultimately, Russian revanchism, and resulted in, among other things, the first full-scale land war in Europe since 1945.

In November 2013, Ukrainian President Victor Yanukovych rejected the terms of the European Union Association Agreement in favor of a $15 billion credit agreement offered by the Russian Federation. Many in the western part of Ukraine had supported the EU deal, as it would have, in their view, secured Ukraine’s future within Europe.

But, as the Europeans, Americans, Ukrainians and Russians knew full well, the association agreement with Brussels wasn’t merely a trade deal. Section 2.3 of the EU-Ukraine association agenda would have required the signatories to:

"...take measures to foster military cooperation and cooperation of technical character between the EU and Ukraine [and] encourage and facilitate direct cooperation on concrete activities, jointly identified by both sides, between relevant Ukrainian institutions and CFSP/CSDP agencies and bodies such as the European Defence Agency, the European Union Institute for Security Studies, the European Union Satellite Centre and the European Security and Defence College."

In other words, the trade deal also included the encouragement of military interoperability with forces viewed, rightly or wrongly, by the Russian government as a threat to Russian national security.

In addition, the EU association agenda required Ukraine to put up barriers to trade with Russia. An alternative proposal put forward by Romano Prodi (former Italian Prime Minister and EU Commission president) would have allowed Ukraine to trade with both Russia and the EU but was rejected by Brussels.

And so, the U.S. lent its support to the post-Maidan government (and the Anti-Terrorist Operation, or ATO, launched in April 2014) against the largely, but of course far from entirely, indigenous uprising in the Donbas. Thus began the first phase of the war, which lasted until the evening of February 24, 2022 and cost 14,000 dead and 1.5 million refugees.

https://responsiblestatecraft.org/maidan-ukraine/