"We don't fight like we used to fight," Trump said. "We used to fight to win. Now we fight for no reason whatsoever. We don't even know what we're doing."
Ohio Gov. John Kasich, who in past debates also has voiced support for increasing ground troops in the region, said the U.S. military needs to "bring all the force you need" to combat the persistent ISIS threat.
"It has got to be 'shock and awe' in the military-speak," he said. "Then once it gets done, and we will wipe them out, once it gets done, it settles down, we come home and let the regional powers redraw the map if that's what it takes."
Texas Sen. Ted Cruz said the next commander-in-chief needs to "do whatever is necessary to utterly defeat ISIS," but he stopped short of specifying how many U.S. ground forces might be needed in the region.
"Right now we're not using a fraction of the tools that we have," he said. "We're not using our overwhelming air power. We're not arming the Kurds. Those need to be the first steps. And then we need to put whatever ground power is needed to carry it out."
On VA, the Republican candidates also criticized the president's handling of recent scandals and reforms. Florida Sen. Marco Rubio connected the high rate of suicide among veterans and problems with delivery of department services to a lack of firings in the department.
"The problem is no one's being held accountable," he said. "Even after we passed [reform] laws, no one's been fired for no outreach... No one's been disciplined. No one's been demoted.
"When I'm president of the United States, if you work at the VA and you are not doing a good job, you will be fired from your job."
The issue of accountability has been a recurring theme for Republican critics of the department over the last two years, since revelations of wait time problems and manipulated records forced the resignation of former VA Secretary Eric Shinseki.
Kasich pushed back against a House-backed proposal to trim some GI Bill benefits to pay for other veterans priorities, but he said the department as a whole "needs to be downsized and spread out."
The past two Democratic debates have shied away from foreign policy issues, but national security still appears to be a key topic in the general election this fall.
On Thursday, Trump repeated his promises to create "a stronger military, much stronger." Rubio called the armed forces "the best military in the world" but added that it "needs to be rebuilt, because Barack Obama is gutting our military."
Next week features five primary election contests, including Ohio and Florida. Both states are considered must-wins for several of the campaigns, and could force some of the candidates from the race if they fall short.
Leo Shane III covers Congress, Veterans Affairs and the White House for Military Times. He can be reached at email@example.com.
Leo covers Congress, Veterans Affairs and the White House for Military Times. He has covered Washington, D.C. since 2004, focusing on military personnel and veterans policies. His work has earned numerous honors, including a 2009 Polk award, a 2010 National Headliner Award, the IAVA Leadership in Journalism award and the VFW News Media award.