The United States is installing new military leadership in Europe at a moment of heightened worries about Russian aggression, doubts about the future of arms control and rising tensions among NATO allies.

These pressures are reflected in stepped-up U.S. military maneuvers in Europe, including the unusual simultaneous deployment last week of two U.S. aircraft carrier strike groups in the Mediterranean Sea. At the same time, the Russians are rattling nerves with talk of fielding new "doomsday" weapons such as a nuclear-armed undersea drone and making moves seen by some as risking escalation of the war in eastern Ukraine.

In ceremonies in Germany on Thursday and in Belgium on Friday, Air Force Gen. Tod Wolters will take over for Army Gen. Curtis Scaparrotti in the dual roles of NATO's Supreme Allied Commander Europe and head of U.S. European Command. Acting Defense Secretary Patrick Shanahan had planned to attend the ceremonies but canceled just hours before his scheduled departure from Washington on Wednesday. A Shanahan spokesman said he decided he should remain in Washington for consultations with the White House and the State Department on the crisis in Venezuela and the situation on the U.S.-Mexican border.

Scaparrotti, who is retiring, spent his tenure's final months dealing with U.S.-Turkey tensions triggered by Turkey's decision to buy a Russian S-400 air defense system. The U.S. and other NATO allies see the deal as incompatible with Turkey's continued participation in the Pentagon's F-35 stealth fighter program, and even its future in NATO. The two countries have been sharply at odds over U.S. support for Kurdish fighters in Syria.

Wolters has made clear his view that the fielding of a Russian air defense system by a NATO ally is unacceptable.

"If Turkey proceeds down a path to procure and operate the S-400, they should not get the F-35," he said at his Senate confirmation hearing on April 2. "I would contend that we all understand that Turkey is an important ally in the region, but it's absolutely unsustainable to support co-location of the F-35 and the S-400."

Wolters, a fighter pilot by training, had most recently served as commander of U.S. Air Forces in Europe and head of NATO's Allied Air Command.

The U.S. dispute with Turkey has the potential to tear the fabric of NATO unity, perhaps achieving a central aim of Russia's strategy toward the West. A Pentagon report to Congress last fall said Turkey's purchase of the S-400 "would have unavoidable negative consequences for U.S.-Turkey bilateral relations, as well as Turkey's role in NATO." Turkey is among NATO member countries in which the United States stores nuclear weapons.

Some in Europe also worry that both Washington and Moscow plan to abandon a Cold War-era treaty that had banned an entire class of nuclear weapons. The U.S. and NATO accused Russia of violating the 1987 Intermediate-range Nuclear Forces treaty; Moscow denies the charge.

In NATO's 70th anniversary year, the alliance also faces a problem that two former ambassadors to NATO call unprecedented.

"NATO's single greatest challenge is the absence of strong, principled American presidential leadership for the first time in its history," Douglas Lute and Nicholas Burns wrote in a report in February for Harvard University's Belfer Center for Science and International Affairs.

"President Donald Trump is regarded widely in NATO capitals as the alliance's most urgent, and often most difficult, problem" because of his open ambivalence about the value of the alliance, they wrote. Trump has accused key members, including Germany, of being freeloaders unwilling to pay for their own defense.

In his final appearance before Congress to present his assessment of security issues facing NATO and European Command, Scaparrotti in March said Russia was his main worry.

"Russia is a long-term, strategic competitor that wants to advance its own objectives at the expense of U.S. prosperity and security and that sees the United States and the NATO alliance as the principal threat to its geopolitical ambitions," Scaparrotti said. "In pursuit of its objectives, Moscow seeks to assert its influence over nations along its periphery, undermine NATO solidarity, and fracture the rules-based international order."

Ukraine is at the center of these concerns.

Although Ukraine is not a NATO member, it has a close working relationship with the alliance. So Russia's 2014 seizure of Crimea from Ukraine and its military intervention in eastern Ukraine are a source of concern in much of Europe. Last week, Russian President Vladimir Putin heightened those concerns by signing a decree to expedite citizenship applications from some Ukrainians living in areas held by Russia-backed separatists. The European Union called Putin's move a sign that he intends to further destabilize the country.

Philip Breedlove, who served as the top NATO commander from 2013 to 2016, said in an interview that Russia poses a multidimensional threat, and that matters only worsened as the U.S. narrowed its dialogue with Moscow in recent years.

"We're moving away from each other, sadly," he said. "Part of that is just because we can't depoliticize the issue of Russia in Washington, D.C." As a result, he added, relations have worsened and solutions have grown more distant. "We need to move forward on a conversation with Russia to have talks that might bring some fruit."