WASHINGTON — President Trump leaves on Friday for a nine-day, five-city foreign excursion, his first trip outside of the United States as the country’s leader and top diplomat.

He doesn’t really want to go.

In recent days, Mr. Trump has groused to several friends that he is not looking forward to leaving his new White House cocoon for high-profile, high-pressure meetings with dozens of world leaders in unfamiliar settings.

At one point, he barked at an aide that he thought his first tour abroad should be only about half as long. He will have to abandon his well-known preference for sleeping in his own bed (or in one at the hotels or golf resorts he owns) as he hops between Saudi Arabia, Israel, Belgium, Italy and the Vatican — all places without a Trump-branded property.

But the trip comes at a critical moment for Mr. Trump’s young presidency. It is his first opportunity to deliver specifics about his “America first” worldview to a global audience that has watched his initial, chaotic months in the White House with a mix of amusement, befuddlement and alarm.

The president’s packed schedule is filled with opportunities for Mr. Trump to slip up, publicly or privately: back-to-back discussions with the leaders of other nations, many of whom are veteran negotiators well versed in issues they care deeply about.

By contrast, it could also serve as a much-needed change of subject for a president besieged at home with an agenda frozen by scandal.

“I’ll meet scores of leaders,” Mr. Trump told Coast Guard cadets on Wednesday as he spoke at their commencement. He pledged that during his global travels he will “strengthen old friendships and will seek new partners.”

Even before heading to Connecticut for that speech, Mr. Trump was brooding in the White House over the latest broadside from the F.B.I. director he fired, James B. Comey. Planning for the foreign trip has proceeded haltingly, with Mr. Trump resisting some of the pageantry that is usually a hallmark of a president’s travels. A visit to Yad Vashem, the Holocaust memorial museum in Jerusalem, was cut short at his request.

Preparation — a standard part of the weeks before a big foreign trip — has also been hit-or-miss in recent days.

As allegations of obstruction of justice and giving secrets to Russia consumed Mr. Trump’s administration, aides sought to focus the president’s fleeting attention on the vital foreign policy issues he will confront and the nuts-and-bolts difficulty of taking the White House around the globe.

Jared Kushner, Mr. Trump’s son-in-law and senior adviser, led briefings, including one on European leaders, last week in the Oval Office. The president conferred with the defense secretary about the Middle East and discussed his Saudi Arabia meetings with his secretary of state. Among the touchy issues in Israel: whether to move the United States Embassy to Jerusalem, something the president has decided not…