At the Kremlin, Putin signed parliamentary legislation incorporating Crimea into Russia, hailing it as a "remarkable event."
At nearly the same time in a ceremony in Brussels, EU leaders sought to pull cash-strapped Ukraine westward by signing a political association agreement with the new Ukrainian prime minister.
The highly symbolic piece of paper is part of the same EU deal that touched off Ukraine's political crisis when then-President Viktor Yanukovych rejected it in November and chose a bailout from Russia instead. That ignited months of protests that eventually drove him from power.
Ukraine's new prime minister, Arseniy Yatsenyuk, a leader of the protest movement, eagerly pushed for the EU agreement.
"This deal meets the aspirations of millions of Ukrainians that want to be a part of the European Union," Yatsenyuk said in Brussels.
The agreement includes security and defense cooperation, he said, though it is far from full EU membership and doesn't include an important free-trade element yet.
But the EU decided to grant Ukraine financial advantages such as reduced tariffs to boost its ailing economy until the full deal can be signed. Those trade advantages are a blow to Russia, which had hoped to pull Ukraine into a Moscow-focused customs union instead.
In exchange for the EU pact, Ukraine's government is promising economic reforms.
"In the long term, the biggest challenge will be to build a strong Ukrainian economy, rooted in strong institutions that respect the rule of law," British Prime Minister David Cameron said at the EU summit.
