Trump's decision to pull out of the Paris climate change accord and the 2015 nuclear deal with Iran threaten Europe's own foreign policy, which was long complementary to Washington's.
President of the European Commission Jean-Claude Juncker also remarked last week that the European Union should replace the United States as a global leader after the latter "as an worldwide actor has lost vigor, and because of it, in the long term, influence".
"Looking at latest decisions of [Trump] someone could even think: with friends like that who needs enemies", wrote Donald Tusk, president of the European Council.
"Europe should be grateful for President Trump because thanks to him we have got rid of all illusions" that the European Union can rely on others for help, Tusk added.
Mr Trump's decision to walk away from the landmark nuclear deal with Iran - to which the UK, France, Germany, Russia and China are co-signatories - is being linked by the European Union with the U.S. administration's refusal to exempt the bloc from steel and aluminium tariffs.
In a scathing attack, former Polish prime minister Tusk compared the Trump administration with Europe's long-time adversaries, Moscow and Beijing.
Trump has effectively hijacked the agenda of the meeting in Bulgaria, which was originally meant to discuss ties with the Balkans, amid the worst rift in transatlantic relations since the 2003 Iraq war.
"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 global relations". "Despite the US hesitation", he said.
With Trump's cabinet being increasingly populated by the likes of National Security Adviser John Bolton and Secretary of State Mike Pompeo, who have long pushed for regime change in Iran, the U.S. position is becoming more at odds with those of Europe, Russia, and China. Tusk told a news conference.
White House National Security Adviser John Bolton indicated on Sunday that the USA would be willing to impose sanctions on any European companies that continue to work or do business with Iran.
Tusk also demanded that Trump grant a permanent exemption to the European Union from threatened tariffs on steel and aluminum, and he rejected as "absurd" any assertion by the that such tariffs could be justified on national security grounds.
"My objective is simple - we stick to our guns", Tusk said of the expected leaders' discussion on trade. He has made us realize that if you need a helping hand, you will find one at the end of your arm.
"We need to bring reality back to this discussion, which is not the case today", he said.