WASHINGTON (AP) — Defense Secretary Jim Mattis has left no doubt that his top priority as leader of the military is making it more “lethal” — better at war and more prepared for it — and yet nothing about the military’s new mission at the U.S.-Mexico border advances that goal. Some argue it detracts from it.

The troops going to the border areas of Texas, Arizona and California are a small fraction of the military’s roughly 1.3 million active-duty members, and the mission is set to last only 45 days. But many question the wisdom of drawing even several thousand away from training for their key purpose: to win wars.

James Stavridis, a retired Navy admiral and former head of the U.S. Southern Command, said the troops should be preparing for combat and other missions, “not monitoring a peaceful border” for the arrival of a migrant caravan of several thousand people on foot, still about 900 miles (1,450 kilometers) away.

“It sends a terrible signal to Latin America and the Caribbean as we unnecessarily militarize our border,” Stavridis, who also served as the top NATO commander, said Thursday. “It places U.S. troops who are fundamentally untrained for the mission of border security and border enforcement into an area of operations, which could cause incidents of a negative character. If we need more border patrol agents, hire them.”

The first 100 or so active duty troops arrived at the border on Thursday, making initial assessment at the McAllen, Texas, crossing. Overall, there are about 2,600 troops at staging bases in the region.

David Lapan, a retired Marine colonel who is a former spokesman for the chairman of the Joint Chiefs and the Department of Homeland Security, said that taking troops away from training and from their families to play a supporting role in border security is unwise.

“It just doesn’t make any sense,” said Lapan, now a vice president of communications at the Bipartisan Policy Center. “This caravan, this group of poor people, including a lot of women and children, doesn’t pose a threat — not a national security threat.”

In line with the Pentagon’s security strategy, Mattis has been focused on improving the combat readiness of a military worn down by the recent years of congressionally imposed budget cuts and the grind of years of war in Afghanistan, including training required for the smaller wars the U.S. has fought since the Sept. 11, 2001 to a “great power” struggle with Russia and China.