EU President Donald Tusk sought to play down the impact of potential U.S. tariffs on steel and aluminum imports as Europe braces for more conflict with President Donald Trump.

Even if the EU doesn’t manage to secure an exemption from Trump’s tariffs, the impact will only affect about 1.5 percent of trans-Atlantic trade, Tusk said in a letter to the European Union’s 28 leaders ahead of a summit in Brussels. By comparison, the U.S. decision to raise tariffs in the 1930s “applied to one third of our trade” and “led to a global trade war,” he said.

EU leaders plan to discuss their response on Thursday. The draft summit statement’s language on tariffs was left blank in a draft seen by Bloomberg, pending a last-ditch effort to secure a waiver as EU trade chief Cecilia Malmstrom and Commerce Secretary Wilbur Ross hold talks in Washington.

Trade is adding to a series of disagreements between the EU and the U.S.—from climate action to Iran’s nuclear accord—that are testing the bonds between the planet’s biggest economies. German Chancellor Angela Merkel suggested last year that Europe can’t take American support for granted, after Trump failed to reaffirm the U.S.’s security commitment.

In his letter to EU leaders, Tusk said while the bloc has no choice but to retaliate, it should not lose sight of the “broader perspective” and must continue to engage with the U.S.

“Transatlantic relations are a cornerstone of the security and prosperity of both the United States and the European Union,” Tusk said.

Russia Tension

Renewed tensions with the U.S. come as the bloc mulls what to do about the poisoning of a former spy in Britain, which the U.K. blames on Russia. EU leaders pledge to “coordinate on the consequences to be drawn in the light of the answers provided by the Russian authorities,” according to the draft statement.

Tusk, who chairs the meetings of EU leaders, didn’t congratulate Vladimir Putin on his re-election this week. By contrast, European Commission head Jean-Claude Juncker sent a warmly-worded letter saying that “positive relations between the European Union and the Russian Federation are crucial to the security of our continent.”

“Our common objective should be to re-establish a cooperative pan-European security order,” said Juncker, who leads the EU’s executive arm.