The president of the European Council has attacked Donald Trump's “capricious assertiveness” after the US leader pulled his country out of the Iran nuclear deal and threatened the EU with tariffs on steel imports. In a letter to EU leaders ahead of their meeting Wednesday night in Sofia, Donald Tusk said that with friends like Donald Trump, the EU could ask who needs enemies. Mr Tusk said that confronted with challenges such as Mr Trump, Russia and China, the EU needed to be united “economically, politically and also militarily like never before” or risk being “a pawn” in global politics. Tonight’s dinner of EU leaders will discuss the bloc’s response to Mr Trump pulling out of the Iran deal, the violence in Gaza sparked in part by the US moving its embassy to Jerusalem and the threatened trade tariffs on steel. Mr Trump has also embarrassed EU leaders over their spending commitments to NATO and appalled the bloc by pulling out of the Paris Agreement of climate change. “Frankly speaking, Europe should be grateful to President Trump,” wrote Mr Tusk, a former prime minister of Poland, “Because thanks to him we have got rid of all illusions.” He said: “He has made us realise that if you need a helping hand, you will find one at the end of your arm.” He added: “We must be prepared for those scenarios, where we will have to act on our own. We have enough potential to rise to the challenge. But what we need is more political unity and determination.” On the looming trade war with the US, Mr Tusk said: “My objective is simple: we stick to our guns. This means a permanent exemption from US tariffs on aluminium and steel if we are to discuss possible trade liberalisation with the US.” On Thursday, EU heads of state and government, including Theresa May, will hold a summit in the Bulgarian capital, which will discuss the expansion of the EU to include Western Balkan countries.