How many troops are serving in America's legacy wars? We still don't really know.

Since late 2017, the Trump administration has withheld releasing troop levels publicly for Afghanistan, Iraq and Syria.

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By Alexa Liautaud

Veteran's Day is a time to honor those who served, but the U.S. public has been given less and less information on how many U.S. personnel are fighting in legacy wars and how many are actually coming home.

Every administration has wrestled with accounting for personnel levels, trying to balance national security with transparency, but since late 2017, the Trump administration hasn't publicly released any levels for Afghanistan, Iraq and Syria.

There are several ways personnel levels are made available to the public. The Defense Manpower Data Center, or DMDC, publishes quarterly data segmented by active-duty, National Guard and Reserve forces, as well as Defense Department civilian employees.

As a timestamped quarterly number, the database isn't comprehensively accurate and excludes some temporary assignments, but in the past, it was often seen as an important tool for researchers, journalists and lawmakers to better understand the military's commitment to certain regions and whether it aligns with U.S. foreign policy.

The Bush and Obama administrations released quarterly data on full personnel numbers for Afghanistan, Iraq and Syria. But nearly two years ago, then the Pentagon simply stopped including the numbers, citing the need to "protect our forces." The database cells, which previously would have included data on America's most high-profile conflicts, are blank.

Although Jim Mattis, then the defense secretary, promised more transparency, President Donald Trump not long after said, "We will not talk about numbers of troops or our plans for further military activities," while announcing a new strategy for Afghanistan. The December 2017 DMDC report included no numbers on the three countries.

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In 2018, several Democrats on the House Oversight and Government Reform subcommittee on National Security penned a letter asking Mattis to reverse the decision.

"This data was publicly available to Congress and the American people throughout the Bush and Obama Administrations and should continue to be so under the Trump Administration," the five Democrats wrote. "In the interest of continued force protection, transparency, and accountability relating to our military presence in key combat zones, we respectfully request that you immediately reverse this policy."

Mattis argued that approximate numbers revealed through other means "appropriately balance the need for transparency with the need to protect sensitive information that could advantage our enemy."

He added: "Of course the Congress is provided fully detailed accounting in classified reporting that is kept current."

Lawmakers still argue, however, that to make effective decisions about policy, funding and national security, the data should be released.

"The Administration's decision to continue to withhold details about U.S. troop levels ... harms Congress's ability to fulfill its Constitutional duties to properly fund and provide effective oversight of ongoing conflicts," one of the signatories, Mark DeSaulnier of California, told NBC News Now in a statement.

And even if committees do receive get classified updates, the numbers can't be used publicly, for example, to inform policy proposals.

"I know that they say that we could get them on classified briefings, but the thing is, classified briefings prevent us from actually sharing that information with the public," Rep. Jimmy Gomez, D-Calif., said in a telephone interview. "If we were developing a policy, we could not reference that information we received. So the public is still sort of in the dark when it comes to the troop levels,"

A spokesman for Republicans on the House Armed Services Committee told NBC News Now that "the committee receives frequent classified updates on troop levels."

The Defense Department told NBC News Now that there were about 13,000 troops in Afghanistan, 5,200 troops in Iraq and 3,000 in Saudi Arabia. It didn't give an answer for the estimate in Syria.

Other opportunities to get even a vague sense of how many personnel are dedicated to particular regions, and whether they match up with public rhetoric, are through the president's biannual War Powers Letter to Congress or through the Congressional Research Service reports.

But the War Powers Letter, a means for the president to "keep Congress informed about deployments of United States Armed Forces equipped for combat," stopped including personnel estimates on sensitive regions as far back December 2017.

And the Congressional Research Service noted in its most recent paper on personnel levels in Afghanistan and Iraq that it didn't have the numbers, either.

Trump has repeatedly said he would be bringing troops home. Ultimately, the lack of information on U.S. personnel levels for high-profile operations puts into question how the U.S. public will ever be able to verify those claims.

This story originally aired on NBC News NOW. Watch live weekdays from 3-11 p.m ET.