Ukraine president Petro Poroshenko calls the signing of the EU trade pact that set off months of war in his country “the Rubicon,” as Ukraine moves to decisively enter the Western orbit economically and diplomatically. He’d better hope he’s wrong about that:

Ukrainian President Petro Poroshenko signed an association agreement with the European Union on Friday, the same deal whose reversal set off a crisis in the nation.

His predecessor’s decision to shun the deal last year and work with Russia instead unleashed deadly strife that led to the ouster of the nation’s President, the loss of Crimea and a pro-Russia separatist rebellion.

Sealing the deal may be the second-most important moment in Ukraine’s history, Poroshenko said, after its independence from Russia.

He said the signing “shows how dramatically things can change in a short time, if the will of the people is strong enough.”

Crossing the Rubicon was a point of no return for Julius Caesar, when he led the Roman army into Rome for the first time ever to complete his seizure of power. That didn’t work out well for Caesar in the end. In this case, it might be the Rubicon for Vladimir Putin too, who has vowed not to let Ukraine spin away from Moscow’s influence. Putin made his displeasure clear earlier today with Ukraine’s decision:

Russia has said that it views the expansion of E.U. ties to its border to be a Western encroachment on a region that has long fallen within the Kremlin’s sphere of influence. E.U. leaders — along with those of Ukraine, Georgia and Moldova — have said that the deal presents no such challenge to good relations with Russia.

“The anti-constitutional coup in Kiev and attempts to artificially impose a choice between Europe and Russia on the Ukrainian people have pushed society toward a split and painful confrontation,” Russian President Vladimir Putin said in Moscow on Friday.

Russian Deputy Foreign Minister Grigory Karasin said the deal would “no doubt . . . have serious consequences,” Russia’s Interfax news agency reported.

At least Ukraine has company in this pact. Both Georgia and Moldova signed the agreement in hopes of expanding markets for their exports and participating in the economic growth of Europe. All three nations are particular sore spots for Russia. Putin fought a war with Georgia in 2008 and ended up breaking two of its provinces away (Abkhazia and South Ossetia) to keep them under Moscow’s thumb, and Russia still has troops in Moldova’s Transnistria district. The people of Transnistria want a return to Russian control, and the war in Ukraine might end up providing Moscow with a path to the territory if Ukraine falls apart.

With all of that said, Poroshenko had little choice but to sign the pact. The fall of the previous Moscow-friendly government run by Viktor Yanukovych was caused by a reversal on an earlier pledge to sign the popular agreement. The Euromaidan protests explicitly demanded a more Western-looking policy and more independence from Moscow. It’s now up to Poroshenko to put the best face on it with Moscow and temper their reaction diplomatically and economically.

Is that possible? The problem in dealing with Putin, Poroshenko says, is that he’s kind of … emotional:

That may be a diplomatic way of saying irrational, but Poroshenko had better not base his policy on that assumption. Everything Putin has done has been in service to his overarching goal of recreating a Russian empire by transforming the surrounding republics into satellites of Moscow, and attempting to destabilize any government that doesn’t cooperate. Putin tried it in Georgia in 2008 and he’s succeeded in Ukraine, at least for the last few months, while taking back the Crimean peninsula. He’d better hope that Putin doesn’t see this as the Rubicon, and that good relations are still possible with a Ukraine that looks in both directions. If not, expect that violent unrest in eastern Ukraine to get a lot worse in the coming weeks.