Two decades ago, before Friedrich Merz came back from the private sector to win the German chancellorship, he accepted an invitation to a gathering of the French Foreign Legion in Corsica. At the last moment, the organizers asked him to arrive on the parade ground not by road or rail, but by parachute.

Mr. Merz, then a corporate lawyer, had never jumped out of a plane. But a fellow attendee recalled recently that Mr. Merz did not hesitate. He made the jump — successfully, but with a bit of a rough landing.

Now 69 and a politician, Mr. Merz is attempting a much more precarious leap with a similar risk of stumbling.

On Tuesday, Mr. Merz, who has no executive experience in government, will become Germany’s 10th chancellor. He will take office at the most challenging time for the nation since the reunification of East and West Germany 35 years ago.

The new chancellor and his coalition government, led by his center-right Christian Democrats, will land in a series of national crises, including a stagnant economy and a frayed relationship with the United States.

An ascendant nationalist party, the anti-immigrant Alternative for Germany, or AfD, which German domestic intelligence just classified as extremist, has surpassed Mr. Merz and his mainstream political counterparts in some polls.