Washington (CNN) Two years into his presidency, Donald Trump is fueling unprecedented uncertainty and anxiety inside the Pentagon. In private conversations over the past month, many of them unsolicited, more than a dozen key military officers, enlisted personnel and senior civilians have expressed worry and concern to CNN. None of the officials have spoken publicly about this, as military law prohibits active-duty personnel from criticizing a sitting president.

It's not just Trump's unpredictable decision making that has officials on edge, it's also his penchant for politicizing the military— something that's come into focus in recent months as he's struggled to fulfill his campaign promise to crack down on immigration and build a border wall. His decision to draw down troops in Syria and his claims that ISIS is defeated have also rankled military commanders who felt it wasn't well thought out.

Some of the highest-ranking officers say there is a new atmosphere of unease inside the Pentagon, particularly among some of the most senior ranks, over the President's inclination to use the military to achieve certain partisan policy objectives. Behind the scenes officials are trying to keep it all at bay. "The amount of time we have to spend making sure our statements and what we say is apolitical is astronomically higher than ever before," one senior military officer told CNN.

If commanders order the troops to perform a mission for reasons that are political — rather than based on national security grounds — the fundamental nature of the US military is changed, several officials worry. That line has already been crossed in the minds of some over the issue of sending troops to the border. It could become even more of a problem should the President decide to declare a national emergency to gain access to Defense Department funds to build the wall.

On top of that, there are also general concerns about the President's foreign policy decisions, particularly his public announcement to pull US troops out of Syria. Wednesday's suicide attack in Manbij in northern Syria that killed American service members raises the question of whether the President's decision to withdraw troops might have resulted in ISIS or other groups seeing a vulnerability and attacking US forces.

Pentagon officials have also been unnerved by requests from the White House National Security Council, which continues to ask the Pentagon for options to attack Iran. Military planners CNN has spoken to say these requests are concerning since there is no real understanding of how Iran might react — or exactly what military objective the Trump Administration is trying to achieve. It's the ultimate worry: the White House orders some type of strike, perhaps against Iranian-backed fighters in Syria, and Tehran retaliates in a counterattack.

Out of the loop

Top military personnel have effectively been no more informed of the President's intentions than the rest of the American public. One example came last Tuesday night as the President gave his prime-time border speech . Before Trump went live at 9pm, questions swirled as to whether he would declare a national emergency to gain access to Defense Department funds to build the wall. Key Pentagon staffers had no insight into what he would say or do. So they prepared a press statement announcing the decision, just in case. They scrubbed it once the speech was done, but the issue hasn't gone away.

In the days since, with no end in sight to the now record-long government shutdown , Trump may still consider circumventing Congress to pay for his border wall by declaring a national emergency. If he does, the Pentagon may have to pay the political price by identifying billions of dollars in military construction funds that could be shifted toward building the wall. Doing so could potentially harm future military readiness, officials say. It could even mean Army Corps of Engineer funds for disaster relief would be diverted.

One military officer told CNN impulsive decision-making by the President is making it harder to plan operations and troop movements with minimum risk to the safety of American forces. The announcement of a sudden withdrawal from Syria was a case in point. "We can jump ten feet high when a President tells us to, but we need to ask when do you want it done? How much risk are you willing to take?"

And since the President tweeted on Sunday that he wants a 20-mile safe zone in northern Syria, there is more confusion. Twenty-four hours later, two defense officials who would be involved in formulating that plan said there was still no understanding of what Trump was talking about. And that was before the Wednesday's suicide bomb attack.

Putting military in political theater

At the same time, there is growing concern among Pentagon officials that Trump will continue trying to use the military as a tool for his own political gain. Of particular concern is his use of partisan rhetoric in front of military audiences— essentially making troops part of his 2020 campaign.

The latest and most explicit public example came Thursday during a speech the President gave at the Pentagon on missile defense, which was laden with partisan references to the border wall and the government shutdown.

Trump blamed what he called Democratic "fringe" party members, who he equated to "the radical left," for a lack of progress toward reopening the government amid negotiations surrounding the border wall. "While many Democrats in the House and Senate would like to make a deal, (House) Speaker (Nancy) Pelosi will not let them negotiate," Trump said. "The party has been hijacked by the open borders fringe within the party. The radical left becoming the radical Democrats."

As Trump made these comments, the room remained silent with no applause. One military official told CNN afterward that it would have been outside the military norms to react to those remarks because of their partisan nature.

Military personnel under regulations are not permitted to participate in political events. But one officer said a 'gray area' appears to be emerging where Trump blows past historical norms, setting up situations imbued with politics. In Thanksgiving holiday phone calls to troops, Trump complained about hot-button political issues such as immigration, federal judges and trade, telling one Coast Guard officer he called that "every nation in the world is taking advantage of us."

During his recent trip to Iraq and Germany, the President engaged with troops by signing red hats emblazoned with his "Make America Great Again" campaign slogan. Military and administration officials insisted it was permissible because the hats were personal items the troops brought to the event. But at the first stop in Iraq, the assembled troops had not been told the President was coming so it's unclear how they knew to bring the hats, or whether they were provided for them, one source said.

White House press secretary Sarah Sanders told CNN the hats were personal items brought by the troops in Iraq and Germany. Sanders said the White House did not distribute them.

General Joseph Dunford, Chairman of the Joint Chiefs of Staff has not spoken specifically in public about Trump politicizing the military but has repeatedly spoken about the ethos of a non-political military force.

The President has had top officials, including his former chief of staff retired Marine Corps General John Kelly, remind him not to involve the military in politics. But there is no evidence he has listened, and with Kelly having resigned in December, there is concern among Pentagon officials that the problem will get worse.

"The President is attempting to portray the military as being on his side," said Mark Hertling, a retired Lieutenant General and a CNN military analyst. "The way the President speaks carries an assumption that the military supports everything he is doing. And that's harmful in several ways."

For starters, says Hertling, it threatens to normalize what has for generations been seen as inappropriate behavior toward the armed forces from the executive branch. It could also erode the trust Americans have in the military for being immune to politics.

All of this could make it easier for the next president to do the same and potentially even more.

Hertling, who retired from the Army in 2013 after a 37 year career in which he rose to serve as the Commanding General of U.S. Army Europe, says he has heard similar concerns about Trump from active duty and retired senior officers, particularly after some 5,000 active duty troops were sent to the border in November to defend against what the President called "an invasion" of asylum seekers from Central America. "You had a lot of soldiers saying 'Why are we doing this? How come the generals aren't standing up for us and saying this is a dumb mission?"

Troops were supposed to return from the border at the end of this month, but on Monday night the Pentagon extended the mission through September 30, 2019.

Dismay over Mattis

Some Pentagon officials are also dismayed over the way the President handled the departure of former Defense Secretary Jim Mattis, who resigned on Dec. 20 in protest over Trump's decision to withdraw troops from Syria. During a Jan. 2 Cabinet meeting, Trump said that he'd "essentially" fired Mattis over a lack of results in Afghanistan.

In recent weeks, Trump has also gone after retired four-star general Stanley McChrystal and Navy Seal Admiral William McRaven, two highly respected officers, who have been critical of Trump's character. Some officials say his subsequent attacks on them are only more unsettling to the rank and file.

Impulsive behavior

Early on many military officers were hopeful about Trump. It appeared he was willing to give the military more autonomy and not micromanage as many troops felt President Barack Obama had done. But they also learned their new commander in chief was impulsive. One of the earliest indications was when he suddenly, via a tweet, said the military would no longer accept transgender persons in the services. It led to immediate court challenges and a months' long review by the Pentagon.

Trump also suddenly said he would stop large scale training exercises on the Korean peninsula, something commanders behind the scenes were not ready for. But even as examples have built up over the last two years, it's what has happened in the last two months or so that is driving the conversation inside the ranks.

With the 2020 campaign cycle approaching, the fundamental concern is the military is being used to veer into partisan territory. This may mean that Acting Defense Secretary Patrick Shanahan and General Mark Milley, who Trump has picked to be the next chairman of the Joint Chiefs of Staff, will be the ones having to keep the Defense Department out of politics. Both men are already critically aware of that challenge, officials say

Across the board, the military historically is well aware that every president uses the armed forces to underscore their own power and role as commander in chief on the world stage. But Trump's impulsive and partisan statements are fundamentally different these personnel say. "We are not a voting bloc for any president," one senior military officer told CNN.

This story has been updated to include political comments President Trump made at the Pentagon on Thursday.