In Tusk's opinion, the greatest threats are the rise of China, the aggressive policy of Russian Federation towards its neighbours, as well as radical Islam which feeds anarchy in the Middle East and Africa.
European Council President Donald Tusk describes the announcements by the new USA administrations as worrisome, putting it with the challenges the EU faces.
Donald Tusk urged EU leaders to take "spectacular steps" to strengthen the bloc's borders and boost economies to restore faith in the European ideal.
"Particularly the change in Washington puts the European Union in a hard situation; with the new administration seeming to put into question the last 70 years of American foreign policy", Tusk said.
Tusk went on to say he believes the European Union now faces an unprecedented threat from the United States.
But he warned that European-American ties were still essential for the future.
Tusk concluded his letter with a warning for the United States: "We should remind our American friends of their own motto: 'United we stand, divided we fall'".
European diplomats said senior national officials and diplomats discussed a possible EU response to Trump at a meeting in Brussels on Monday.
Another said that because the full USA administration was not yet in place, Europeans should be cautious: "No government in Europe can respond in a coherent manner to this series of orders and tweets", the diplomat said.
From the Estonian capital, Tallinn, the European Union president claimed that the first weeks of Donald Trump's presidency are contributing to the "highly unpredictable" outlook for the bloc, CBS News reported this morning.
The statements have raised concerns in Europe, as have Mr Trump's views on trade.
They will then meet without British Prime Minister Theresa May to discuss the future after Brexit, and preparations for a summit in Rome in March to mark the 60th anniversary of the EU's founding treaty.
With Trump calling for renegotiated trade deals that favor the U.S., Tusk advocated for the European Union to protect its citizens and businesses and remember "that free trade means fair trade". That, diplomats said, risked breaking European Union rules on equal treatment for European Union citizens.