ROME — When President Trump landed in Saudi Arabia last Saturday on his first foreign trip, he brought with him a $110 billion arms deal. When he arrives at the NATO summit in Belgium on Thursday, he will bring mostly questions, many of them about the war in Afghanistan.

The Pentagon is pushing to reinforce the Afghan army with up to 5,000 more American troops. Some administration officials expected Mr. Trump to make a decision on a deployment before the NATO meeting, which would have laid down a marker for the other alliance members.

But now the president’s decision has been delayed, officials said, after an intense debate erupted in the West Wing over the wisdom of pouring more soldiers into a 16-year-old conflict.

Mr. Trump, they said, wants to gauge what other NATO members are willing to contribute to the military effort before he makes any commitment. The United States is already spending $3.1 billion a month in Afghanistan — a number that aides say weighs on a president who has spoken about the need for greater burden-sharing from NATO allies.

The thornier question is how Mr. Trump will reconcile the split between his war cabinet — led by Defense Secretary Jim Mattis and Lt. Gen. H.R. McMaster, the national security adviser, who both served in Afghanistan — and his political aides, among them his chief strategist, Stephen K. Bannon, who argue that a major deployment would be a slippery slope to nation building, something Mr. Trump has always shunned.

The president’s son-in-law and senior adviser, Jared Kushner, has not voiced an opinion on specific troop proposals, officials say. But he is critical of existing American policy in Afghanistan and views his role as making sure his father-in-law gets “credible options.”

Mr. Trump has given the military more leeway than his predecessor, President Barack Obama, signing off on aggressive early moves like commando raids in Yemen and the Tomahawk missile strike on Syria. But he has said little about Afghanistan, either as a candidate or as president, and his aides rarely include it on their list of foreign policy priorities.

“The questions they have to ask are: Is that additional force decisive? Are we going to win? Can we force a political settlement?” said Jack Keane, a retired Army vice chief of staff, who said he told Mr. Trump during the transition that the current policy in Afghanistan was failing.

“I don’t think it is unusual that they would be having a debate,” General Keane continued, “particularly given that we have a 16-year war. U.S. policies have largely driven us to a 16-year war.”

Senior Pentagon officials are broadly supportive of the American commanders’ request for several thousand additional troops in Afghanistan, but they acknowledge they face persistent questions, if not outright opposition, to the plans from certain corners of the White House.

In addition to the cost and the worries about nation building, critics doubt that President Ashraf Ghani will be any more effective than his predecessors in curbing the rampant corruption in his country that has siphoned off billions of dollars in American aid in the last decade.

On Sunday, Mr. Trump met Mr. Ghani in Saudi Arabia. A White House official said he commended the Afghan president’s “leadership in Afghanistan on fighting terrorism and implementing key reforms,” and praised the bravery of Afghan troops. But the statement said nothing about more
American support.

National Security Council staff members are peppering their Pentagon colleagues with questions, expressing fears of costly, open-ended American troop commitments. Senior White House officials are demanding greater commitments from NATO allies before the United States sends more troops, officials said.

In what is perhaps a sign of this internal wrangling, Mr. Mattis told reporters on Friday that he had not yet sent a formal troop recommendation to the White House. “What I’ve done is I’ve gone to Afghanistan,” he said. “I’ve met President Ghani, I’ve met with the NATO representatives and I’ve met with our commanders in the field.”

With the NATO meeting this week, Mr. Mattis said the recommendations that he and Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, were putting together would be delivered “very, very soon.”

The international security force assisting the Afghan army has about 13,000 troops, of which about 8,400 are American soldiers. Pentagon officials said that 3,000 to 5,000 additional troops, including hundreds of Special Operations forces, could be sent.

Such a deployment would allow American advisers to train and assist a greater number of Afghan forces, and place American troops closer to the front lines at lower levels of in the chain of command. Those figures are broadly consistent with advice Gen. John W. Nicholson Jr., the top
American commander in Afghanistan, gave Congress in February.

General Nicholson has warned lawmakers that the United States and its NATO allies faced a “stalemate” in Afghanistan, where the general has said he already faced a shortfall of a “few thousand” troops. Other commanders and experts on military planning say even that prognosis is too optimistic.

“The reality is that the Taliban have the initiative,” General Keane said. “They have the momentum. They attack when they want, where they want, and the outcome is usually successful for them.”

Deploying more troops would cost billions of dollars, and there is no guarantee of a clear victory over the Taliban, which controls much of the territory outside Afghan cities. The United States failed to force successful negotiations when it had 100,000 troops in Afghanistan.

Yet without a strong American military role, the Taliban and more extreme groups like the Islamic State’s Afghan wing and the Haqqani Network would most likely gain ground, undermining Mr. Trump’s promise to destroy Islamic extremists, according to military officers and analysts.

“The Afghan army is taking the brunt of the fight against these transnational terrorists and the Taliban,” General McMaster said in a recent briefing. “And so we are working with our allies to figure out what more we can do to have a more effective strategy.”

As a commander in Afghanistan in 2010, General McMaster headed a task force that tried to root out endemic corruption in the government. That experience, former colleagues say, has persuaded him of the need for the United States to stay involved in the country. One of his current aides described his view of Afghanistan as “tempered optimism.”

Pulling back would put Mr. Trump at odds with generals he embraced and turned to for national security advice. It would also fly in the face of recommendations for more troops from the State Department, which is seeking to stabilize Mr. Ghani’s government, and the nation’s spy agencies, which rely on the military for security to carry out their covert missions inside Afghanistan and across the border in Pakistan.

There are echoes, in the current debate, of the first year of the Obama administration, when Mr. Obama’s generals — led by David H. Petraeus and Stanley A. McChrystal — pushed for a large troop surge, rankling his political aides including Rahm Emanuel and David Axelrod, who complained that the generals were boxing in their commander in chief.

“Trump has the same challenge Obama did: If you walk away from Afghanistan, it’s going to be a strategic disaster,” said James Jay Carafano, a senior fellow at the Heritage Foundation who advised Mr. Trump’s transition team. “Even if Trump’s instincts are to say, ‘This isn’t the kind of military operation I’d like do,’ he’s caught with the strategic reality.”