Washington (CNN) When the defense ministers of NATO countries meet in Brussels this week, there will be a notable absence: a Senate-confirmed, permanent United States Secretary of Defense.

The acting secretary and expected permanent nominee , Mark Esper, will be there, just as he was present at the White House last week as President Donald Trump and his national security team deliberated launching a military strike against Iran . But it's not the same.

An acting defense secretary lacks the political capital and internal confidence of a permanent one. The Senate confirmation vests a secretary with explicit and implicit powers -- from the ability to recruit and set long-term policies in the Pentagon to the power to communicate authoritatively with foreign counterparts and members of Congress.

This absence was on full display last week as the President mulled striking Iran, ultimately pulling back at the very last minute . Though Esper was in the room, along with his predecessor Patrick Shanahan, who served as acting secretary from more than six months before stepping down over the weekend, neither was in position to play what several experts and former Pentagon officials say is the crucial role of a Defense secretary -- to reconcile the goals and agenda of the administration with the realities presented by the military.

"It's just not good. You want to have a fully empowered Secretary of Defense," Eric Edelman, the former under secretary of Defense for policy in the George W. Bush administration, told CNN. "You need to have a civilian perspective, one that brings some political sense."

There hasn't been a permanent, Senate-confirmed Secretary of Defense since James Mattis resigned from the position on December 31 -- a record amount of time for the Pentagon to go without a leader.

Compounding that, the Department is in the midst of an unprecedented period of leadership turmoil. Esper is rapidly being briefed on the complex responsibilities of the acting secretary. But thanks to federal vacancy law, Esper will have to step down as acting secretary once he is formally nominated for the permanent position.

When that happens, the Secretary of the Navy, Richard Spencer, is likely to become the new acting secretary and will have to go through a similar training regimen -- only to hold the acting job for the brief time before Esper would be expected to be confirmed by the Senate.

The prospect of three acting secretaries of defense within a few weeks could have been avoided had the President acted more quickly to nominate either Shanahan or a different candidate for the top job. Instead, the Pentagon has been in a period of leadership limbo for half a year.

Pentagon on autopilot

This is not to say that the Pentagon can't function without permanent civilian leadership. The Defense Department consists of massive, largely self-sustaining bureaucracies -- the Joint Chiefs of Staff, the military branches and the operational commands, for starters -- meaning the Pentagon's daily operations won't shut down or even noticeably slow without a permanent secretary. In many ways, the Department runs by itself.

But there's a reason it's designed to have a strong, Senate-confirmed civilian at the top. It's as much about political decisions as it is operational ones. The secretary's job is to set and communicate policy so that military officials can execute that policy.

But an acting secretary is limited in that capacity. For starters, no one's clear how long they'll be in the job. They're barred from recruiting and retaining lower-level officials. They can't institute long-term reforms, and it's hard for them to command authority both in Congress and on the international stage. Having an acting secretary in charge of the Pentagon isn't a big problem for a few weeks. But when it lasts for six months, the ramifications start to pile up, according to a half dozen defense experts and former officials who spoke to CNN.

"Leaving the role acting for such an insanely long time has hamstrung the ability of the Pentagon to operate in a normal manner, not just in time of crisis, but also on long-term plans," said Peter W. Singer, a defense expert and senior fellow at the New America Foundation.

Mark Hertling, a retired Army commander and a CNN contributor, says the uniformed officers at the Pentagon he speaks with tell him the turnover and instability among the Pentagon's civilian leadership has left many in the military brass feeling as if they are on their own.

"When there's a lack of guidance from civilian officials, people are taking a little bit of risk," said Hertling. "Now that you have the constant churn, where it's one after another, it does really put some additional stress on the military guys."

The stress can be particularly acute for military officers on temporary assignment to the Pentagon. One retired officer tells CNN these servicemembers, accustomed to "getting things done" in the field, have been frustrated by how the bureaucratic process has slowed these past few months due to the unclear chain of civilian command.

"When you have that much uncertainty, people lower down the food chain will just sit on issues," Gary Schmitt, a national security strategy scholar at the American Enterprise Institute.

The prolonged acting status for top Pentagon officials may be emboldening US adversaries. Several Republican supporters of the President in Washington have told CNN in recent days that they fear the power vacuum at the Defense Department may have partially fueled the current crisis with Iran.

No more Mattis

Mattis's resignation at the end of 2018 came at a crucial time for the Pentagon and the administration's defense policy. The US military began drawing down its presence in Syria in January. Eight US soldiers were killed in Afghanistan between January and April as the President continued to pursue withdrawal from that country, too. And in February, the President held a second summit with North Korean leader Kim Jong-un seeking to curb that country's nuclear program.

In the first half of 2019, there has also been unrest in Venezuela, a few close encounters with Russian ships and planes , plus the regular defense budget request process -- all before the crisis with Iran intensified in recent weeks.

"It's an increasingly complex time with a whole bunch of problem sets," Hertling said. "Because of the crisis du jour, there is the shifting of resources and staffing. You have a bunch of people shifting between problem sets."

"It's a zoo," he added.

Shanahan cut short

Even after Trump decided to nominate Shanahan for the full-time role of Defense secretary, the former Boeing executive and Mattis deputy was still widely seen as temporary, even within the administration.

That created a vacuum on Trump's national security team that's been filled by Secretary of State Mike Pompeo and national security adviser John Bolton. Many Washington observers clearly saw Shanahan as a placeholder.

"When you are in a Cabinet meeting or the Situation Room...everybody is polite but there is this unwritten thing where the permanent guy's views are going to be given more weight than an acting," said Tom Spoehr, a former Pentagon official and the director of the Center for National Defense at the Heritage Foundation.

That perception was also alive on Capitol Hill, where lawmakers worried that Shanahan lacked the stature of Mattis, a retired four-star Marine general. The acting secretary failed to impress when appearing before congressional hearings and even many Republicans were more resigned than enthusiasti c about Shanahan's impending nomination.

That had clear implications for the Trump agenda. In one appearance in March before the Senate Armed Services Committee, Shanahan struggled to defend the Space Force proposal. Even Republican lawmakers predisposed to support the President came away from the hearing sounding skeptical.

Edelman, the former Pentagon official, says he is concerned with how the lack of a permanent Defense Secretary has exacerbated an existing problem for the Department -- namely, the shortage of civilian officials who bring political considerations to the table. The lack of authority for an acting secretary to fill important roles maintains that policy vacuum that the military brass are not equipped to fill.

"We already had a civilian-military imbalance at the Defense Department, and Trump is making it worse," he told CNN. "It has to be remedied quickly."



UPDATE: This story has been updated with a timeline of events since Jim Mattis resigned as Secretary of Defense.