Defense Secretary Pete Hegseth has summoned hundreds of the military’s most senior officers and their enlisted counterparts to attend a gathering on Tuesday at a military base in Virginia.

Military officials and historians say the event is without precedent in size and scope.

Word of the meeting trickled out from the Pentagon last week, but the Defense Department has offered few details about why Mr. Hegseth called such a large assembly or what he hopes to accomplish with it.

The lack of candor has fueled suspicion and anxiety among admirals and generals, who are flying from their operational posts around the world to attend.

What has the Pentagon said about the gathering?

Officially, not much.

Some Pentagon officials have been told that Mr. Hegseth is expected to make it a pep rally of sorts, exhorting the assembled senior officers to support his plans to center what he calls “the warrior ethos” from the top ranks on down.

A senior official said last week that one goal was to get fighters excited about the new posture of the department.

Which officers are expected to attend?

Pentagon officials have said that all admirals and generals in command positions have been invited, along with their senior enlisted leaders.

This would almost certainly include leaders of the geographic combatant commands like U.S. Indo-Pacific Command and U.S. European Command that direct military operations across separate swaths of the globe.

Based on the description of the meeting given by Pentagon officials, it could also include the three-star officers who lead Navy fleets and Army corps commanders.

Where is the event being held?

The meeting will take place at Marine Corps Base Quantico in Northern Virginia.

Who else will be in attendance?

As of Sunday: President Trump.

It was not immediately clear whether he planned to give a formal address.

Mr. Trump appeared not to know about the plans for the high-level military meeting when he was asked about it in the Oval Office on Thursday. But he quickly said it was good that Mr. Hegseth had organized it.

In an essay for The Bulwark, Mark Hertling, a retired Army lieutenant general, wrote that he expected the assembled military service members to offer a polite but restrained reception in line with longstanding tradition that military service members not act in a way that would show partisan political support in public.

“I hope the loudest message they send is no message at all — only that they have the quiet, disciplined silence of professionals who know their oath is to the Constitution, not to a man,” he wrote.

How unusual is this type of meeting?

Very.

Security concerns alone would usually preclude the public advertising of a meeting among such high-ranking officers in a single place.

How are those security concerns being addressed?

The attendees will be driven to Quantico in buses that will depart from separate locations, according to a military official.

The official spoke on the condition of anonymity because they were not authorized to speak publicly about security measures for the event.

How much will it cost?

Many of the officers will be flying to the Washington area from bases around the world, but the total costs are unknown.

On Saturday, two Democratic senators on the Armed Services Committee, Tammy Duckworth of Illinois and Mazie Hirono of Hawaii, criticized the meeting as costly and disruptive.

“This gathering would represent an unprecedented concentration of senior military leadership in one location simply to hear you speak about standards and ethos at significant cost and with potentially serious security implications,” the senators said in a letter to Mr. Hegseth.

“For an administration obsessed with rooting out waste, this abrupt, time and resource-intensive meeting of our military’s top commanders,” the senators continued, “is absurd.”