Leaders are gathers to explore options to keep the Iran nuclear deal afloat after US President Donald Trump made a decision to withdraw from the deal and reimpose sanctions on Iran.
Mr Tusk said: "Besides traditional political challenges, such as the rise of China or the aggressive stance of Russian Federation, we are witnessing today a new phenomenon: the capricious assertiveness of the American administration. Looking at the latest decisions of President Trump some could even think, 'With friends like that, who needs enemies?'" Thanks to him we got rid of all illusions. "We realise that if you need a helping hand, you will find one at the end of your arm".
"My objective is simple - we stick to our guns", Tusk said of the expected leaders' discussion on trade.
French President Emmanuel Macron has tried to charm Trump but that failed to prevent Washington last week from abandoning the Iran deal, which seeks to curb Tehran's nuclear programme in exchange for sanctions relief.
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. Trump also warned his European allies he would impose stiff sanctions on EU companies who either do business in Iran or who opt to honour legally binding business contracts with Iranian companies.
Tusk, a former prime minister of Poland who now presides over one branch of European Union policymaking, went full zen in his angry description of Trump's effect on Europe.
Washington and Brussels have been at loggerheads over a number of issues since Mr Trump was inaugurated in January 2017, most recently a trade war of aluminium and steel tariffs and the Iran nuclear deal.
Tusk added that Europe must protect the "transatlantic bond", but be prepared to act on its own, with "more political unity and determination".
"Juncker and [Foreign Affairs chief Federica] Mogherini will present various variants of how to protect the interests of European operators after the departure of the U.S. from the nuclear agreement", a senior EU official told reporters on May 15 ahead of the EU-Western Balkans Summit in Sofia.
Mr Tusk said the USA 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.
Mr Tusk said he wanted the 28 leaders over the next 24 hours to reconfirm that the European Union would stick to the deal as long as Iran did. Therefore U.S. tariffs can not be justified on the basis of national security. "The deal is good for European and global security, which is why we must maintain it", he said.
"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.
This move sparked discontent among the key USA trading partners, especially the EU.