WASHINGTON – The Obama administration has made tough choices to reform the Pentagon that have left the Department of Defense positioned to meet the challenges of the future, Secretary of Defense Ash Carter argued in a new essay summing up the last eight years for the department.

The letter, which comes in at almost 10,000 words, was posted on Carter's Medium blog. It is part of a series of memos from each of the department secretaries of the Obama administration, which were released en masse this week.

Included are many of the ideas that have been staples of Carter speeches over the last year, including the need to modernize the Pentagon, ongoing operations against the "parent tumor" of the Islamic State in Syria and Iraq, and developing the "Force of the Future" to keep the all-volunteer force strong in the coming years.

"As this Administration took office in early 2009, the country had just experienced another deadly year in Iraq and Afghanistan. The Department of Defense was therefore necessarily focused on large-scale counterinsurgency operations to defeat extremist networks in those countries," Carter wrote in his opening to the piece, before arguing the case that the Obama administration "effectively adjusted its defense strategy, shifting from a focus on irregular warfare and counterterrorism to a return to full-spectrum combat readiness and directing a responsive and versatile military that can prevail across the full spectrum of operations."

Combined with the impact of the 2011 Budget Control Act, the Pentagon instead invested in a "pursuit of a smaller yet more technologically advanced and capable military that is ready for the threats of today and the challenges of tomorrow," Carter wrote.

"While the next Administration will continue to be challenged by an evolving security environment, I am confident that our military is up to the task of protecting our nation in the years ahead," Carter concluded. "The President-elect can count on them to continue to execute all their duties with the excellence our citizens know they can expect; may God continue to bless them, and continue to bless the United States of America."

Carter is expected to further sum up his legacy as secretary of defense with a Jan. 11 appearance at the Center for Strategic and International Studies.

Sec. of Defense Ash Carter at Reagan National Defense Forum

US Secretary of Defense Ash Carter took the stage at the Reagan National Defense Forum and delivered what amounted to a whirlwind summary of his two years in the job, casting it in the context of how he wants to leave the department in good shape for his successor.