Following a slow-drip series of admonishments about sanctioning Russia over its aggression in Ukraine — actions widely perceived as tantamount to inaction — European Union leaders agreed Sunday night to provide more than half a billion dollars’ worth of armaments and other aid to Ukraine in its ongoing battle against Russian invading forces.

Branding it a watershed moment, the European Commission president, Ursula von der Leyen, said, “For the first time ever, the E.U. will finance the purchase and delivery of weapons and other equipment to a country that is under attack.” 

The E.U.’s foreign policy chief, Josep Borrell, added to the sense of newfound eurocratic swagger: “The taboo that the E.U. was not providing arms in a war, yes we are doing it.… This war requires our engagement in order to support the Ukrainian army.”

Early indications are that the decision took some prodding. Speaking on Sky News Monday morning, the British secretary of defense, Ben Wallace, said that “it took quite a lot of effort” to persuade the E.U. to ramp up its response to the rapidly escalating military crisis at its doorstep. America, he asserted, has been sending arms to Ukraine since at least 2015, after Russia annexed Crimea. Furthermore, “Britain is now the center of coordination” for arms deliveries to Ukraine, Mr. Wallace added. 

The momentum to arm Ukraine seemed to be building from all quarters except perhaps Brussels. Mr. Borell’s breathless explications — via an “emergency videoconference” of E.U. foreign ministers — came a day after Germany said it would begin shipping lethal aid to Ukraine and on the day that Greece sent planes packed with missile launchers and Kalashnikov-style rifles to Ukraine. That military cargo arrived at Poland’s Rzeszow-Jasionka airport several hours before Brussels’s big announcement. 

Anti-tank and anti-aircraft weapons are what Ukraine needs the most right now, the British defense secretary said, and the E.U.’s plan should expedite the delivery of such defensive weapons as well as protective gear and fuel. 

The funding will come from the European Peace Facility, an off-budget instrument that according to the European Commission “allows the E.U. to do more and to act more swiftly to support global peace and security through actions with military and defense implications.” The mechanism allows the E.U., which was established in the aftermath of World War II as an economic alliance known then as the European Coal and Steel Community, to reimburse its member states for their arms transfers to Ukraine’s military.

In another development that underscores the complexity of Europe taking truly unified action, Finland announced Sunday that it might send weapons to Ukraine’s army directly. Helsinki has so far limited its assistance to sending defensive equipment such as bulletproof vests and helmets to Kyiv. While it is a full member of the European Union, Finland — which shares an 830-mile border with Russia — is not a member of NATO. Shipping arms out of the country to Ukraine would mark a stark departure from Finland’s policy that interdicts the export of weapons to war zones.