As Democratic presidential candidates debate the merits of Medicare for All, a Green New Deal or free college, a chorus of scolds from across the political spectrum will chime in to tell you we can’t afford it.

All these ambitious policies of course will come with a hefty price tag. Proposals to fund Medicare for All have focused on raising taxes. But what if we could imagine another way entirely?

Over 18 years, the United States has spent $4.9 trillion on wars, with only more intractable violence in the Middle East and beyond to show for it. That’s nearly the $300 billion per year over the current system that is estimated to cover Medicare for All (though estimates vary).

While we can’t un-spend that $4.9 trillion, imagine if we could make different choices for the next 20 years.

We’ve identified more than $300 billion in annual military savings alone that we could better invest in priorities like Medicare for All, working with a national grassroots movement called Poor People’s Campaign.

Cutting military spending this way presents its own tremendous obstacles. Yet the exercise, however aspirational it may seem, also shows how ambitious proposals are still within reach — if we make different choices.

The $4.9 trillion we’ve spent on wars in Iraq, Afghanistan, Yemen, Syria and elsewhere grows to $5.9 trillion once we add in future care for veterans of those wars. That’s more money than the G.D.P. of every country in the world other than the United States and China.

These are unwinnable wars. After nearly 18 years in Afghanistan, the Taliban now “controls or contests” more Afghan territory than at any time since before the invasion. One Army study of the Iraq war noted that “an emboldened and expansionist Iran appears to be the only victor” of the U.S.-launched war there.

Despite the obvious downsides, Pentagon officials first requested $165 billion in war funds for 2020, although even they admit that only $66 billion of that is actually related to fighting wars — the rest covers regular operations.

The government doesn’t just shell out for our own military — we pay significant sums for foreign militaries, too. The $14 billion provided to foreign militaries in 2017 is more than five times the budget of the United Nations.

With more than 800 bases and installations in more than 90 countries, our military is ready to step into any conflict at any time, and often that’s just what it does. Our bases in Germany, established after World War II, serve as a launchpad for missions in the Middle East and Africa, where the U.S. has quietly been escalating its presence for years.

That’s a terrible model. Just imagine if the U.S. still had thousands of troops in Afghanistan in 2076.

Our overseas bases come with all sorts of costs. Just shipping private vehicles overseas for military personnel costs around $200 million each year. In the context of the $716 billion military budget, that doesn’t sound like much, but it’s 50 percent more than the U.S. spent on international disaster prevention and preparedness in 2017.

Closing half or more of our foreign bases could actually enhance our national security by defusing unnecessary tensions and discouraging ill-thought-out interventions. And it would raise about $90 billion for things like Medicare for All. What are we doing with that base in Aruba, anyway?

The United Nations has called for nuclear disarmament and the prohibition of nuclear weapons, and at least 70 countries have signed on to a total nuclear weapons ban. The U.S. agreed to negotiate nuclear disarmament in the Non-Proliferation Treaty, though it has failed to follow through on that commitment.

The U.S. now has 4,000 deployed or stockpiled nuclear weapons, plus 2,500 more retired weapons awaiting dismantling. Our nuclear stockpile amounts to almost half the world total of 15,000.

A total nuclear weapons ban would mean new treaties and would take years to negotiate. But if we could get there, we could save roughly $43 billion each year on weapons, delivery systems and upgrades, according to calculations based on data from the Department of Defense, the Congressional Budget Office and the Department of Energy. That’s roughly the same amount we’ve allocated in federal hurricane aid for Puerto Rico. Even canceling current plans for nuclear modernization and simply continuing on our current path could save about $20 billion per year.

Private contractors consumed fully half the Pentagon budget in 2018, raking in more than $364 billion.

The F-35 jet fighter is a prime example. Over its lifetime, the F-35 is estimated to cost $1.5 trillion — more than the G.D.P. of Australia.

This is a plane with more than 900 performance deficiencies, according to a recent government report. Among the problems is a dangerous night vision defect in the plane’s high-tech pilot’s helmet, which costs $400,000, four times more than a typical helmet for other fighters, like the F-16.

Like so many weapons programs, the F-35 program is a congressional darling, but not because of military necessity. A group of more than 100 members of the House of Representatives wrote a letter calling for the purchase of 24 more F-35s than the 79 that President Trump has already called for. With parts of the F-35 manufactured or assembled in 350 congressional districts, it’s no mystery what is driving the enthusiasm for the plane.

Our proposed cuts to production and operation of the F-35 total $14.7 billion — more than the military budget of Iran.

President Trump’s border wall has been estimated to cost as much as $25 billion. The sum budgeted for this year alone, at $9 billion, is more than the budget for the Environmental Protection Agency. Rather than using this money to build a wall for a nonexistent emergency, why not put it toward Medicare for All?

Likewise, the planned Space Force is a novelty that the administration first promoted with a logo bearing the phrase “Mars Awaits.” While military spending related to Mars is currently unknown, the $400 million that the administration has requested for its Space Force is a down payment for a project that would quickly reach many billions of dollars.

Under Medicare for All, we would no longer need a separate Pentagon system to provide health care for troops and their families. This now costs $33 billion a year. This item takes us well past our goal of saving $300 billion.

Hundreds of thousands of active-duty military members are also assigned administrative jobs that could be done by civilians at lower cost. Transferring just one-quarter of those jobs to civilians could conservatively save $3 billion, enough to cover the current value of Affordable Care Act tax credits for health premiums in 46 states (but not Florida, California, Texas and North Carolina).

Many military experts are likely to disagree with some or even all of these options. And no doubt there are other equally sensible options for cutting military spending. (Our cuts total $2,807 for every American household.)

Remaking our military as a truly defense-based institution, rather than a war machine and A.T.M. for private contractors, will require major changes. It’s a project that can’t happen overnight, and it will need serious planning and wiser uses of some of our $50 billion surplus to ensure both U.S. security and that people leaving military service find new jobs in our economy. That’s no excuse for continuing to spend hundreds of billions in ways that make our world more dangerous and deny us the ability to seriously invest in things like jobs, health care, education and all that makes our lives better.