Now the leaders of France and Germany have told him that in no uncertain terms:
The ceasefire agreement for eastern Ukraine has just been recast to put the onus on Poroshenko, rather than on Russian President Vladimir Putin.
Going into Friday's negotiations with French President Francois Hollande, German Chancellor Angela Merkel and Putin in Paris, Poroshenko was looking belligerent.
He had just delivered a hard-hitting speech at the United Nations, entirely devoted to Russia's depredations against his country.
His interior minister, Arsen Avakov, was boasting that the Ukrainian National Guard had "finally" received U.S. sniper rifles and anti-tank grenades.
Ukraine's Other War
French diplomat Pierre Morel, who has been in close contact with Moscow and the Russian-backed rebels in eastern Ukraine, prepared a plan for the Paris meeting of the four leaders to approve.
According to Morel's proposal, Ukraine would need to pass a special law setting out rules for the local elections in the rebel-held areas of Ukraine.
That was a cunning way to defuse a time bomb planted under the Minsk cease-fire deal reached last February.
Back then, Russia and its proxies agreed to an election under Ukrainian law by the end of the year, but they were clearly not prepared to hold it under the current legislation, which doesn't differentiate the rebel areas from all the others in Ukraine.
They were threatening to hold their own polls in mid-October, something that might cause the war to reignite.
Poroshenko, however, swept the French diplomat's suggestion aside as "Mr. Morel's personal opinion."
He was going into the meeting to demand Russia abide by the Minsk ceasefire, cancel what he called "fake elections" and return control of Ukraine's eastern border to Kiev by the end of the year.
He underestimated the determination of France and Germany to get the Ukrainian matter out of the way in the most efficient manner possible.
After five hours of talks in the Elysee Palace, the Morel plan was imposed on Ukraine in a form more beneficial to Putin.
First, Ukraine must design the special election law in consultation with Moscow and the separatists.
Then, it will have to pass it and amnesty the separatist leaders so they can run for local legislatures.
In 80 days' time, after the passage of the law, the election should be held.
Then, if international observers declare it acceptable, Ukraine is supposed to regain control of its border with Russia.
Hollande told reporters after the talks that wasn't likely to happen this year, because of the need to draft the legislation and properly prepare the election.
This is a slap in Poroshenko's face.
It's almost politically impossible for him to push a Moscow-approved election bill through Ukraine's parliament.
Poroshenko has had trouble getting the legislature even to approve a tame constitutional amendment allowing for a special status of the rebel-held regions; riots broke out outside the parliament building during the vote and police suffered casualties.
Trying to sell election rules favorable to Moscow might mean the breakup of Ukraine's ruling coalition and perhaps snap elections likely to produce a parliament less favorable to Poroshenko.
"Paris has once again confirmed that in defending Ukraine's national interests Ukrainians have no allies but themselves," commentator Pyotr Shuklinov wrote bitterly on Liga.net.
"Berlin and Paris decided to play the role of arbiters. Neither is willing to take decisive action to end the war in the center of Europe."
In addition to being granted an extension of the Minsk agreement, Putin will have the pleasure of watching Poroshenko squirm as he tries to water down the Morel plan -- or doesn't try hard enough to get the election bill approved.
Any failure in that effort would give Putin a more or less permanently frozen conflict with which to distract Ukraine's resources and destabilize Poroshenko's government.
And if the ball remains in Ukraine's court, economic sanctions against Russia may also be lifted -- Putin has made sure since last month that the war zone remains quiet.
At the same time, Poroshenko will be in trouble with Europe's leaders, who would become ever more suspicious of his intentions.
The way Merkel and Hollande see it, Poroshenko should be interested in working to reintegrate the rebel-held areas into Ukraine, which would mean contesting the election and, in case of an almost certain defeat, working with the winners.
That's the European way of doing things; trying to enlist outside support to defeat the separatists is not, especially when Europe has plenty of problems of its own.
Poroshenko can count on meaningful support only if he shows a commitment to do difficult things that would bring Ukraine closer to Western governance models:
Achieve tough political compromises and implement painful reforms.
So far, the Ukrainian president hasn't delivered on either front.
His country is still hopelessly corrupt and gripped with infighting among oligarch clans, despite the government having created no fewer than five new anti-corruption bodies.
In the absence of true deregulation and tax liberalization, economic growth remains elusive -- the International Monetary Fund has just lowered its growth forecast for Ukraine this year to a decline of 11 percent, from the 9 percent it predicted in June.
The war the Ukrainian government is losing now is against mismanagement, overregulation and graft.
That's just what Putin wants.
His bet in the eastern Ukraine local election, if it ever takes place, won't be on the rebel field commanders but on local oligarchs who ran the region before the 2014 "revolution of dignity."
Through them, he will hope to exert both economic and political influence on Kiev.
He can afford to wait; time is running out for Poroshenko, not for him.