ANALYSIS: Trump's sobering decision as commander-in-chief

As a businessman, political candidate and now president, Donald Trump has been consistently critical of the handling, cost and duration of America's longest war.

Earlier this month, he declared that the military's protracted involvement in Afghanistan was "unacceptable." The top American general there has recently described the conflict after 16 years as a "stalemate."

But President Trump has struggled to decide on a cohesive strategy of his own to turn things around. Tonight he will unveil his long-awaited plan for Afghanistan -- the moment the war will truly become Trump's War.

We know Trump has been conflicted over a path forward in Afghanistan since taking office. He is leery of open-ended conflict, previously called for a full pullout of U.S. troops, and long complained about the trillion-dollar costs to taxpayers and lost young American lives.

More than 2,400 Americans have been killed in Afghanistan since 2001. Eleven have been killed so far this year, including one soldier who died last week.

But Trump also loves to win, and worries about being labeled the president who lost. Defense Secretary Jim Mattis, national security adviser H.R. McMaster and other top brass want Trump to essentially stay the course. They have argued to him that withdrawal now would create a vacuum in Afghanistan, allowing the Taliban to take hold and establish a safe haven for terror groups.

There are currently 8,400 American troops on the ground in Afghanistan. Mattis and the generals are recommending an additional 4,000 to help break the stalemate.

Whatever Trump has decided has not leaked to the media in detail. But several reports suggest the president will take Mattis' advice and likely impose some new conditions of his own. As we've seen in other foreign negotiations, Trump favors mandating greater burden-sharing by America's partners -- both in financial costs and resources. Look for that in this new strategy.

If Trump approves more troops to continue this 16-year-old war, it is undoubtedly the most consequential decision he's made as commander in chief in terms of impact on America's troops and their families.

Unlike the Syrian air strikes and the surgical special-ops raids he's ordered, the Afghanistan strategy decision could put thousands more American lives on the line in harm's way. No doubt a sobering decision for Trump -- perhaps the most sobering decision he's had to make as president so far.

ABC News' Alexander Mallin and Luis Martinez contributed reporting.