Secretary of Defense Jim Mattis visited Capitol Hill last week and gave Congress an important message.

He told lawmakers that President Trump had authorized him to decide how many American military personnel should be deployed in Afghanistan. This is good news if there is to be anything like a sustainable peace there.

For too long, the military has been unable to send more personnel than the amounts decided arbitrarily by President Obama, who was obsessed with building a legacy as a peacemaker rather than with doing what would have been effective. This forced the Pentagon to keep a low troop presence in Afghanistan, but it was not a path to stability, let alone peace. The caps were akin to Obama's declaration that the Iraq War was over, when all that was over was Washington's commitment to the benighted people of that country.

Obama's approach was the height of strategic incompetence. Elevating domestic politics over strategic needs, he prevented the military from carrying out his own strategy and attaining his own goals.

But where Obama was strategically illiterate, Mattis is the opposite. A keen thinker renowned for his grasp of military theory, history, and philosophy, Mattis can bring new energy to America's long war in Afghanistan. His strategy, due to be presented to Congress next month, is likely to involve three key components: combating Islamic State, strengthening the Afghan government, and pressing the Taliban and other insurgents toward serious negotiations.

To implement that strategy, he'll need flexibility in force deployments. Afghan security forces lack the military support capabilities they need against the Taliban. They are assisted by 8,400 American miilitary personnel and 5,000 more from other NATO states. But it's not enough.

Speaking in February, John Nicholson, the commanding General in Afghanistan, explained that he needed "a few thousand" more American personnel.

He has many tasks and too few people to undertake them simultaneously. With more forces, he can provide an agile and lethal partner to Afghan forces.

Military commanders always want more forces. But by delegating deployment authority to Mattis, arguably the most capable person in the Trump administration, the president ensures he has expert eyes assessing Nicholson's requests. That will make it harder for critics to accuse Trump of bad strategy.

Another benefit is that it will put NATO states under pressure to do the same. While Trump is deeply disliked by European NATO leaders, Mattis is deeply respected. If he, rather than Trump, is seen to be shaping strategy in Afghanistan, there'll be more NATO buy-in. It will also deny NATO leaders the excuse of ignoring Washington's requests simply because they dislike Trump. If NATO states continue to refuse help, Mattis' efforts will buttress the Trump administration should it, for example, decide to move American military bases out of Germany and into Poland (which meets NATO's defense spending target).

It's easy to misunderstand the nature of Trump's action and regard the ceding of troop levels to the Pentagon as a step away from the possibility of peace. It was similarly easy to see Obama's troop caps as a peaceful move.

But what's easy often isn't true. When President Ronald Reagan spoke at Normandy, he carefully chose the words with which he honored the Army Rangers who sat before him. "These are the boys of Pointe du Hoc," he began, before continuing: "These are the men who took the cliffs. These are the champions who helped free a continent." Steadily dialing up the moniker, from boys, to men, to champions, Reagan finished this riff with the highest praise: "These are the heroes who helped end a war."

Ending a war and making peace possible, is what a soldier like Mattis sees as his calling. Obama wouldn't give the generals the tools they needed to do so. Trump has just done so. By handing Mattis the keys, he's given our soldiers, sailors, airmen and Marines the chance to end a war.