President Trump was typically self-absorbed in his tweet on Wednesday celebrating the 242nd birthday of the United States Army. “Proud to be your commander-in-chief,” he proclaimed to the soldiers.

Yet, when it comes to the actual life-and-death responsibilities of the commander in chief — overseeing America’s vast war machine and sending men and women into conflict — Mr. Trump seems more like the delegator in chief. The latest evidence was his decision this week to give Defense Secretary Jim Mattis the authority to determine troop levels in Afghanistan, which could lead to an increase of as many as 5,000 troops, if proposals favored by Mr. Mattis and his generals go forward.

Mr. Mattis has acknowledged to Congress that the United States-led coalition is “not winning” in Afghanistan. It is not at all clear that adding 5,000 more troops — a roughly 50 percent increase over the current troop level of 9,800 — can make a difference, especially when the administration has yet to confront the basic problem of ensuring public safety and the larger political and economic issues that must be part of a comprehensive strategy to resolve the conflict.

What such a decision would do is reverse the drawdown President Barack Obama put in place and set a new policy of expanding involvement in a war that has already dragged on for 16 years, cost thousands of American and Afghan lives and consumed billions of dollars.