U.S. Defense Secretary Jim Mattis will only play a supporting role to President Donald Trump at this week's NATO summit — an event that by definition is focused on heads of state from the trans-Atlantic alliance.

But Mattis' small part belies his high standing among NATO allies, which has only risen as they become increasingly bewildered by Trump's policies on trade and Iran, and anxious about his outreach to Russian President Vladimir Putin, European diplomats, officials and experts say.

In recent months, it has become clear that Mattis has a limited ability to influence Trump, who is increasingly confident in his own foreign policy instincts as he settles into his presidency.