The EU's 28 leaders are gathering in the Bulgarian capital for a dinner discussion on Wednesday on how to salvage the Tehran accord and European business dealings with Iran, from Trump's sanctions to how to avoid a trade war in an escalating tariff dispute with the United States.
"Following last week's announcement of the United States withdrawal from the Joint Comprehensive Plan of Action with Iran, we need a united European front", said Tusk who arrived in Sofia to participate in the EU-Western Balkans Summit.
Speaking on Wednesday, Tusk said he was "grateful" for Trump because he had made the European Union realise "if you need a helping hand, you will find one at the end of your arm".
At the outset of a summit of European leaders whose agenda items, point by point, have to do with the flames of crises that many Europeans see as ignited by President Donald Trump, Tusk ripped into what he called "the capricious assertiveness of the American administration" over issues including Iran, Gaza, trade tariffs and North Korea.
In a scathing attack, former Polish prime minister Tusk compared the Trump administration with Europe's long-time adversaries, Moscow and Beijing.
The EU is hoping that Iran will continue to obey the rules of the agreement as long as enough of the economic benefits promised in the deal are preserved.
"The role of the EU is absolutely essential", he said, "And so my appeal here in Brussels is for the the European Union to be more and more united, more and more effective, more and more present and for its voice to be more and more heard in worldwide relations". "He made us realize that if you need a helping hand, you will find one at the end of your arm". Therefore U.S. tariffs can not be justified on the basis of national security.
While European officials have spoken out against Trump's trade actions before, the language used by Tusk marks a new tone in the relationship.
White House National Security Adviser John Bolton indicated on Sunday that the US would be willing to impose sanctions on any European companies that continue to work or do business with Iran.
The violence in Gaza will also be on the agenda as it was "linked to the bigger question of the consequences of the decisions of Donald Trump", an European Union official said.
It was crucial, he said, that the European Union stuck to its guns and refused to talk trade with the U.S. until it received a permanent exemption from punitive tariffs on steel and aluminium imposed by Washington on the grounds of national security.
This move sparked discontent among the key United States trading partners, especially the EU.
"I have no doubt that in the new global game, Europe will either be one of the major players, or a pawn. We need to bring back reality in this discussion, which is not the case today".