The bloc's leaders will hope to present a united front against Mr Trump in response to his decision on the Iran nuclear deal and trade tariffs.
Listing Europe's traditional challenges, ranging from the expanding power of China to the belligerence of Russia, Tusk said: "We are witnessing today a new phenomenon, the capricious assertiveness of the American administration". "Looking at the latest decisions of President Trump, someone could even think with friends like that who needs enemies", Tusk told reporters in the Bulgarian capital Sofia before European Union leaders met to discuss a "united front" on Trump. But frankly speaking, Europe should be grateful to President Trump. "He has made us realize if you need a helping hand, you will find one at the end of your arm".
Tusk's remarks, just hours before he was to chair a meeting in Bulgaria of the 28 leaders of the world's biggest trading bloc, underscored the widening gulf in EU-U.S. relations.
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.
The Iran nuclear deal, or Joint Comprehensive Plan of Action (JCPOA), was signed in July 2015 after Tehran, Russia, the United States, United Kingdom, China, France, and Germany agreed on a detailed, 159-page agreement that established a series of limitations on Iran's nuclear programme that would keep the Islamic Republic from developing weapons-grade plutonium in exchange for the lifting of crippling worldwide sanctions.
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.
Meanwhile the European Union is still trying to win exemptions from tariffs on steel and aluminium exports. 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. "But at the same time we must be prepared for scenarios where we'll have to act on our own".
European leaders have scrambled to salvage the 2015 agreement in which Iran agreed to halt its nuclear program, a landmark deal that many Europeans see as essential for their security.
Mr Tusk said the United States appeared to be hesitating in hitting European companies doing business in Iran with sanctions, but the bloc still needed to be ready to protect its interests. Therefore US tariffs can not be justified on the basis of national security.
In dinner talks in the Bulgarian capital of Sofia, EU leaders will be briefed on possible incentives to keep Tehran in the nuclear agreement, even after a key player like the USA pulled out.
"We knew that there was a great risk, that this decision to move moving the embassy would bring less security, bring tragedies, and sadly we were right", he said.