President Trump vowed on Monday to beef up the American military presence in Afghanistan, a strategy that promises to extend the longest war in U.S. history and add billions to its financial cost.

One current estimate pegs the conflict's total cost at $841 billion. That comes from Anthony Cordesman, the Arleigh A. Burke Chair in Strategy at the Center for Strategic and International Studies.

Cordesman, who served as a consultant to the Departments of State and Defense during the Afghan and Iraq wars, says that figure includes Trump's budget request for next year.

Related from CNN: Afghanistan: 16 years, thousands dead and no clear end in sight

Other estimates place the 16-year cost in the trillions of dollars because they measure a broader range of factors.

For instance, Neta Crawford, a co-director of the Cost of Wars Project at Brown University, has estimated that total war spending in Iraq, Afghanistan and Pakistan since 2001 is approaching $5 trillion. Of that, roughly $2 trillion is attributable to Afghanistan. That includes some future cost obligations.

But even that higher figure leaves out some key expenses, such as the future costs of interest Americans will owe for the money borrowed to finance the war in Afghanistan. That alone could add trillions of dollars to the total tab.

While the U.S. has a history of wartime taxation to finance military conflicts -- albeit uneven -- that tradition was broken with the wars in Afghanistan and Iraq, according to tax historian Joe Thorndike. Not only did Congress not pass a tax to finance the efforts, it opted instead to pass the Bush tax cuts.

Related: The U.S. already spends more on defense than any other country

The $2 trillion also doesn't include future spending on the Department of Veterans Affairs related to Afghanistan or the money paid by states and localities for services provided to returning vets.

Estimates vary widely because there is no clearly delineated, uniform way that money spent on wars is allocated or counted by the White House or Congress.

And, of course, no financial estimate can offer a measure of the true cost of war -- the loss of human life on all sides as well as the physical and psychological disabilities suffered by those who survive -- whether military or civilian.