A recent report by Vox suggests that a full troop withdrawal from Afghanistan by May 1 may be “off the table.” What a shame.

Not only would changing course from his predecessor break one of Biden’s campaign promises — his website still calls for an end to the “forever wars in Afghanistan and the Middle East” — it’s also a huge missed opportunity.

The casualties and lives lost over the last two decades are no doubt the greatest hardship to come from the United States overstaying its welcome in Afghanistan. But there’s another cost that is also quite significant — the financial one.

Brown University’s Costs of War project has documented in painstaking detail the sheer amount of money the United States has spent in Afghanistan, and it’s not a pretty picture. As the authors summarize, “Through Fiscal Year 2020, the United States federal government has spent or obligated $6.4 trillion dollars on the wars in Afghanistan, Pakistan, and Iraq.” About half of that total can be attributed to Afghanistan alone.

Yet, perhaps more important than what has already been spent is how much could be saved. One of the issues with war spending is that costs don’t just stop when the military conflict comes to an end. Expenses compound over time, yes, because of accrued interest, but also because of ongoing expenses like physical and mental health care for injured veterans. For that reason, figuring out potential savings can be a challenge.

By the end of 2019, the Trump administration had done little — rhetoric notwithstanding — to alter the United States’ troop presence in Afghanistan. About 14,000 troops remained on the ground, far below the Obama-era peak, but also significantly above the 8,700 that remained when Trump took office.

A report I authored, Rethinking Afghanistan, estimated at the time that a full withdrawal from 14,000 to no troops had the potential to save between $210 and $386 billion over four years. This included not just direct war costs, but also increases to the base and Homeland Security budgets, additional veteran and disability obligations, and interest on all of the above. With more than 80 percent of ground troops now withdrawn from their 2019 level, much of this cost savings is well on the way to being realized.

But not all of it. That estimate also assumed that the drawdown would continue, leaving only a residual force of 1,000 troops — or none at all. All bets are off if Biden decides to ignore the May 1 deadline, or worse, reverses course entirely and increases ground troops in the name of ensuring the country’s safety.

Rerunning the numbers from that previous analysis gives a sense of what this course change would cost over the next couple of years. Assuming that the Biden administration continues on the current course and maintains a presence of 2,500 troops on the ground during the next two years, the result would be additional fiscal costs of between $33 and $49 billion, relative to a full withdrawal. Compared with a residual force of 1,000 troops, maintaining the status quo would still cost an additional $7-10 billion over the next two years.

Of course, it’s also not a guarantee that Biden would keep troop levels the same. For another point of comparison, it’s worth looking at a cost estimate if the Biden administration decides to double the current troop count to 5,000, maintaining that level through FY 2023. In that scenario, our actions could cost as much as $66-$98 billion, relative to a full withdrawal, and $39-59 billion compared with a more minimal troop presence. All of these costs include direct war expenses, projected increases in the base budget, more disability and mental health treatment for veterans, and of course, expected interest costs from borrowing to make these expenditures.

It’s easy to forget how as simple an action as keeping troops on the ground can have dramatic impacts at home. In addition to increased casualties of Americans and Afghans alike, putting off plans to leave the country also carries substantial fiscal cost at a time when the federal budget is already stretched to the max by the U.S.’s response to the pandemic.

New Defense Secretary Lloyd Austin, speaking in his first public press conference last week, stated that “We are mindful of the looming deadlines, but we want to do this methodically and deliberately.” Point taken, but as Washington Post columnist David Ignatius put it recently, even if the cost of keeping a small force “appears relatively low… it’s Biden who will have to write a letter of condolence to the family of the first American who dies on his watch.”

After 20 years, the time has long come to save our lives and our treasure, and withdraw from Afghanistan once and for all.