WASHINGTON — President Trump’s declaration that “we will win” in Afghanistan came with bellicose language about taking “fast and powerful” retribution against militants and a promise to succeed where other presidents have failed to resolve the nearly 16-year-old conflict.

Left unclear by the Trump administration was the answer to a question that has stumped Washington, including the two previous commanders in chief, since the United States became involved in the war there: What does winning look like in Afghanistan?

For Mr. Trump, winning looks a lot like a very long war.

“From now on, victory will have a clear definition,” Mr. Trump said on Monday during his prime-time televised address on his strategy for Afghanistan. “Attacking our enemies, obliterating ISIS, crushing Al Qaeda, preventing the Taliban from taking over Afghanistan, and stopping mass terror attacks against America before they emerge.”

But hours before Mr. Trump spoke, the president’s top advisers offered a far different view of success, arguing that the ultimate solution to the conflict in Afghanistan was a settlement between the Afghan government and the Taliban, who would theoretically be beaten down by the Americans and forced to the negotiating table.