“No better friend; no worse enemy” is the motto of the 1st Marine Division — the oldest, largest and most-decorated division in the United States Marine Corps. It also happens to be a superb description of the unit’s former commander and the nation’s 26th Defense secretary, James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE.

No one has been a more faithful friend to U.S. national security, or a more committed adversary to those who threaten it, than Jim Mattis. Few military or civilian leaders have possessed as firm a grasp of America’s rightful role in the world, or a clearer understanding of evolving threats, than this man of extraordinary integrity and intellect.

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All of which complicates the decision of how to replace him at the Defense Department. It may be the most momentous choice that President Donald Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE will make in the remainder of his term, one that will determine the nation’s security now and long into the future.

As a Marine general and Defense secretary, Mattis innately understood that America must remain engaged in the world and exercise its global leadership, if it is to remain influential and prosperous in the 21st century and if the world is to enjoy the stability and development so instrumental to long-term U.S. national security.

From his enlistment in 1969 and commission as an officer three years later, throughout his many posts and commands (including the Joint Forces and Central commands), Gen. Mattis combined a deep knowledge of history and keen strategic vision with a profound love of country and affection for those who sacrifice in its defense.

Though highly decorated, his most valued honor undoubtedly was the enduring respect and esteem felt by the young men and women who served under him, including combat in Afghanistan and Iraq.

The legendary story about him, as a brigadier general, dressing for guard duty on a Christmas Day so that a young lieutenant could spend the holiday with family, is not only true but just one such instance in an exemplary life of leadership. It was not enough that his rank and authority demanded respect; he saw it as a profound duty and privilege to be earned every day. He regularly reached out to junior officers, not only to get to know them personally but to learn from them — creating a better force, and making himself a better commander. He assured the servicemembers under his command that, just as faithfully as they had the nation’s back, he had theirs.

Gen. Mattis also was instrumental in modernizing military doctrine. He was a primary architect of the U.S. Army/Marine Corps Counterinsurgency Field Manual. That 2008 rewrite placed emphasis on the strategic, operational and tactical importance of U.S. troops understanding the culture where they are engaged in conflict, of making friends as well as defeating adversaries in order to achieve military and geostrategic objectives. This is among the most important points of emphasis in military doctrine to emerge in the 21st century, one the nation ignores or deemphasizes at its peril.

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Such personal qualities and professional skills are why Gen. Mattis was superbly qualified to serve as Defense secretary and why Congress, for only the second time in the country’s history, waived the required seven-year waiting period for a former military leader to head the department. (The first time was for Gen. George C. Marshall.)

Among his accomplishments as Defense secretary was modernizing the National Defense Strategy to better posture the military in confronting near-peer threats of the future, helping accelerate the battlefield effort to defeat ISIS, and bringing a steady, reassuring hand to the Pentagon.

Thus, his eventual successor has large shoes to fill and must learn from his example to ensure the nation’s defense in complex, pivotal times.

It will be imperative for the incoming secretary to demonstrate to the troops, the services, the department, the president, Congress, the nation and our allies that he or she possesses the skills and characteristics to succeed. Among those:

A clear understanding and ordering of the myriad of threats to the nation’s security;





A strategic vision and strong imagination for meeting them;





The humility and will to build strong, constructive working relationships with the Joint Chiefs of Staff chairman;





The ability to lead and manage the government’s most complex bureaucracy and continue the progress of the joint-force concept;





The willingness to strongly advocate the department’s views to the president and to Congress, while faithfully and morally executing acts of Congress and lawful presidential decisions and directives; and





The wisdom to keep servicemembers, the services, the president, Congress, the press and the public properly informed and consulted.

The challenges the new secretary and his team will face are monumental. Some are obvious, such as leading our ongoing combat operations around the world, while others are not as well understood or properly prioritized. Among the latter category, five warrant special mention:

Reining in skyrocketing personnel costs that threaten to undermine the effectiveness of the force;





Defining and exercising the Defense Department’s role in modernizing U.S. global engagement to prevent instability in vulnerable hotspots that can burst into regional and global conflict;





Reinforcing America's role within the network of international security alliances;





Assisting in the development of a comprehensive national cybersecurity strategy, while securing the Defense Department’s digital networks against adversaries working to compromise U.S. command-and-control systems, threaten critical defense infrastructure and destroy our qualitative military edge (QME);





Translating battlefield victories against ISIS into long-term strategic success by preventing the organization’s regeneration or reiteration, particularly in strategically vital areas, even as the defense establishment refocuses on near-peer threats; and





Fulfilling the department’s vital threat-reduction role, particularly in countering the proliferation or use of nuclear, chemical and biological weapons of mass destruction.

We should honor and thank Secretary Mattis for his life of service. Yet, as a patriot, instead of kudos his most fervent hope is most certainly for the nation to fully support his successor in the vital work that should be above partisan rancor and division — providing the absolute best civilian leadership of our services and servicemembers in protecting the United States and its global interests.