The 28 anxious EU leaders are gathering in the Bulgarian capital for discussions over dinner on Wednesday on how to salvage the nuclear deal and European business dealings with Iran from Trump's sanctions and how to avoid a trade war in an escalating tariff dispute with the United States.
This emerges from a letter to EU leaders from European Council President Donald Tusk.
Tusk even compared the United States administration to Europe's traditional foes Moscow and Beijing as he launched his broadside hours before a dinner of the 28 leaders in Sofia where they will discuss the issue.
European diplomats have sought to play down expectations of Tuesday's meeting, stressing the enormous challenge of finding a way around USA sanctions punishing foreign businesses trading with Iran, which have global reach.
"I want the European Commission to be given a green light, so it is ready to act when European interests are effected", he added.
On Saturday, Merkel told Europeans that the USA had made potential peace in the Middle East "even more difficult" and warned that "the harder the problem is, and the easier the solution is claimed to be, the more suspicious and critical everyone should be".
Tusk said EU leaders would use the summit to discuss the Iran deal along with innovation, the European future for the western Balkans, and Trump's trade policies.
On trade, Tusk said the USA move to impose trade penalties on national-security grounds was "absurd", since the European Union is a core American ally and does not represent a threat.More news: Trucker Putin opens Russian bridge to annexed Crimea
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"Here again, unity is our greatest strength and my objective is simple - we stick to our guns".
The European Commission is refusing to open talks on wider trade terms with the USA, including import tariffs on cars - a bugbear of the United States president - until it receives a permanent exemption from punitive taxes on European steel and aluminium.
He added, "The EU and US are friends and partners".
This move sparked discontent among the key USA trading partners, especially the EU. "We need to bring back reality in this discussion, which is not the case today".
Shortly after Trump's election, Tusk had joked that his European colleagues had taken to calling him "Our Donald". Tusk told a news conference.
After listing more of their proposals, Nigel finished: "So the whole European project is crumbling, the third biggest country in the eurozone is in open revolt against the whole system and all Donald Tusk can do is say a few sad words about Donald Trump".
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". "He made us realize that if you need a helping hand, you will find one at the end of your arm". In order to be the subject and not the object of global politics, Europe must be united economically, politically and also militarily like never before.