Deputy Defense Secretary Patrick Shanahan is now in charge at the Pentagon, taking over as acting defense secretary Monday from outgoing Defense Secretary Jim Mattis.

The formal handover of control took place in a conference call between Mr. Mattis and Mr. Shanahan at 11:59 p.m. Monday and was required because the secretary position is second in the chain of command after the president, and thus it was important to make clear who is in charge of American military forces.

Joining Mr. Mattis in making an exit from the Pentagon is press secretary Dana White, who gave no reason for her departure as Mr. Mattis’ chief spokeswoman. Ms. White’s replacement is Charles E. Summers, a Pentagon public affairs official who is now acting assistant to the defense secretary for public affairs.

Several aides to Mr. Mattis are remaining in the office of the secretary of defense during the transition.

They include retired Rear Adm. Kevin M. Sweeney, who was Mr. Mattis’ chief of staff. Mr. Sweeney is considered a key player in defense policy and operations.

Peter F. Verga is staying on as deputy chief of staff to the acting secretary of defense. Marine Corps Maj. Gen. George W. Smith, who was recently promoted to lieutenant general, is continuing as the senior military assistant to Mr. Shanahan.

Mr. Shanahan has designated David L. Norquist, the Pentagon comptroller and chief financial officer, as acting deputy secretary of defense.

Mr. Shanahan said in a statement Tuesday that the Defense Department remains focused on safeguarding the nation under the direction of President Trump.

“We have deep respect for Secretary Mattis’ lifetime of service, and it has been a privilege to serve as his deputy secretary,” he stated. “As acting secretary of defense, I now look forward to working with President Trump to carry out his vision alongside strong leaders including the service secretaries, the Joint Chiefs of Staff, the combatant commanders, and senior personnel in the office of the secretary of defense.”

Pentagon spokesman Tom Crosson said Mr. Shanahan will continue pursuing Mr. Mattis’ three lines of emphasis as acting secretary. Those priorities include a multiyear plan of increasing the lethality and agility of American military forces and improving warfighting readiness.

As part of that effort, the Pentagon is investing more than $50 billion over 10 years to rebuild U.S. nuclear forces. Conventional forces will be modernized, and special operations and irregular warfare forces will remain a core military tool.

The second line of effort will be to strengthen foreign alliances and international partnerships with nations that share America’s vision for a free and democratic world, including the NATO alliance, partnerships with Southeast Asian states and the coalition battling the Islamic State. Mr. Mattis resigned after Mr. Trump announced he was ordering the withdrawal of U.S. forces working with local forces in fighting the Islamic State in Syria.

The third Pentagon priority for Mr. Shanahan is to bring much-needed business reforms, a strong suit for the former Boeing executive who is no doubt familiar with the broken system of weapons procurement in the department.

Mr. Shanahan will be making calls in the coming days to key foreign defense chiefs and will testify before Congress in the coming weeks on the U.S. defense posture, Mr. Crosson said. He appeared beside Mr. Trump at his first Cabinet meeting Wednesday.

In his farewell letter to Pentagon employees Dec. 31, Mr. Mattis quoted from President Lincoln’s one-sentence letter to Gen. Ulysses S. Grant two months before the Civil War ended: “Let nothing which is transpiring, change, hinder or delay your military movements or plans.”

“It has been my high honor to serve at your side,” Mr. Mattis stated. “May God hold you safe in the air, on land and at sea.”

PUTIN TOUTS HYPERSONIC MISSILES

President Vladimir Putin and other Russian officials have been bragging about Moscow’s development and coming deployment of a hypersonic missile capable of penetrating U.S. missile defenses.

Mr. Putin on Dec. 26 observed the latest flight test of the new Avangard hypersonic missile, a maneuvering weapon that flies many times the speed of sound and thus is hard to track and counter. The Avangard is a winged glider that was test-fired from the last stage of a ballistic missile; the exercise reportedly was successful.

The launch took place from Russia’s Dombarovsky missile complex in the Ural Mountains and hit a target at the Kura proving ground on the Kamchatka Peninsula in the Russian Far East, some 3,700 miles away.

“This is a great success and a big victory. This is a wonderful, excellent gift for the country for the new year,” the Russian leader said after the test.

Mr. Putin claimed that the missile is invulnerable to current air defense and anti-missile systems.

“It was a hard and time-consuming work which required breakthrough solutions in principal areas, and all this was done by our scientists, designers and engineers,” he stated.

The weapon system will be deployed next year with a special regiment.

The Avangard will be carried atop SS-19 ballistic missiles and reaches hypersonic speed before gliding to a target without a power source at high altitude but below near-Earth space. The rapid development of the Avangard is unusual, and U.S. intelligence agencies have said Moscow lagged behind the efforts of China and the United States to develop similar high-speed missile gliders.

In October 2013, a CIA-based Open Source Center report said, “Russia probably is years away from developing a hypersonic missile or vehicle, despite officials’ optimistic statements.”

The Russian development of hypersonics is based on Moscow’s fears of falling behind U.S. development of the high-speed missiles.

In November, Michael Griffin, undersecretary of defense for research and engineering, said the Pentagon needs to speed up work to protect ships and bases from hypersonic missile attack.

“This is a threat with a much reduced signature, by a factor of 10 or 20 times less than the strategic missile threat with which we’ve dealt in the past, so we need to be somewhat closer to the action,” Mr. Griffin said in a speech. “This is not a threat in the future. We know that China in particular has done dozens of tests of hypersonic systems. Russia has done fewer, but still impressive. This is a threat that our adversaries are developing.”

Mr. Griffin said the United States has done research on hypersonic flight but chose not to weaponize it.

“We didn’t think that the world needed necessarily more weapons, and the United States doesn’t seek adversaries. Other people have sought to be our adversaries and are succeeding at that, and so our choice is how and when and where do we respond, but we have to respond. So if they choose to weaponize these capabilities, then we will have no choice but to respond in kind.’

According to Russian Deputy Prime Minister Yuri Borisov, the Avangard flies 27 times the speed of sound, or more than 20,000 miles per hour. Such speeds make guidance and control very difficult.

Mr. Borisov told Russia’s Zvezda television channel that the latest tests “have shown that it has reached speeds close to Mach 30. Practically, at these speeds, no anti-missile can knock it down.”

Former Russian Defense Minister Sergei Ivanov said in a state-run Russian television interview that the Avangard changes course and altitude constantly during flight through the atmosphere, chaotically zigzagging on the way to its target and frustrating technical efforts to track and target the missile with missile defenses.

Mr. Ivanov said Russia has a stockpile of several dozen Avangards that are stored unfueled, giving them long shelf life. The glider will be deployed in existing missile silos in order to reduce deployment costs.

“The Avangard has cost hundreds of times less than what the U.S. has spent on its missile defense,” Mr. Ivanov said.

The former defense chief said development of the hypersonic missile began in 2002 after the United States withdrew from the 1972 Anti-Ballistic Missile Treaty.

• Contact Bill Gertz on Twitter at @BillGertz.

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