On Wednesday, as families continued to bury their loved ones killed at Marjory Stoneman Douglas High School in Parkland, Florida, President Trump suggested that one answer to gun violence in schools might be to post armed veterans to deter would-be attackers—and stop them with return fire if necessary.

The idea, suggested a week ago by Sean Hannity on Fox News, resonated with many who believe in the myth of a “good guy with a gun,” and admire the American military too.

But for many practical reasons, it’s a ludicrous idea. Although their firearms familiarity is better than the national average, most troops and veterans don’t have the skill to carry out the kind of duty Trump suggests. It would take multiple troops to secure a single school, making this effort incredibly costly. And even if the logistics made sense, arming veterans to guard schools would turn the occasional incident into a firefight, likely killing or wounding many more in the crossfire.

Most Americans have shot a gun at some point in their lives. All service members do so as part of their basic training. For some, such as infantry soldiers or Marines, this training lasts for weeks and includes training on how to shoot under duress. However, most troops do not serve in the infantry or special operations or military police fields—assignments that emphasize firearms proficiency. The typical service member learns to qualify on a rifle or pistol range in basic training, actually shooting targets for just a handful of days. Upon arriving at a military unit after basic training, most troops qualify with their assigned pistol or rifle just once or twice a year, maybe shooting 50 or 100 rounds each time at paper or plastic targets in a series of timed drills. This type of semiannual qualification helps maintain basic proficiency—but it does not create the kind of close-quarters marksmanship necessary to prevail in an urban firefight. Only certain military units (like special operations, infantry, or law enforcement) shoot often enough, and practice in the right ways, to develop that level of skills.

Relatedly, most troops don’t actually carry their weapons when on base. Military pistols, rifles, machine guns, and ammunition remain locked up in secure arms rooms for most units, coming out only for training ranges or exercises. On base, military police carry weapons just like civilian police officers do, but they are the exception. When deployed to Iraq or Afghanistan, and on large bases there, most service members carry unloaded weapons, as a safety precaution to make sure they don’t accidentally shoot their buddies.

These protocols may differ for elite troops with more extensive firearms training, but they suggest that the average veteran probably doesn’t shoot well enough to effectively stop a school gunman. Moreover, troops don’t train to fight alone, or serve as security guards, and no school can be effectively secured by just one armed guard. It would likely take a squad of nine to 11 well-trained guards—and possibly more for large schools with athletic facilities or multiple points of entry. The Department of Education estimates there are approximately 131,890 public and private K-12 schools in America, meaning it could take 1.3 million armed guards to secure the nation’s schools. That’s equal to the size of the entire active duty U.S. military today. But to deploy that many guards, it would take an even larger force to recruit, train, equip, and manage all those personnel, with the costs running into the hundreds of billions of dollars if military compensation is any guide.

Even if veterans were good enough with weapons, and it was feasible to deploy them at every school, this is still a flawed idea. The “good guy with a gun” scenario is a dangerous fantasy that doesn’t work in practice. It doesn’t work, in part, because running to the sound of gunfire is a brave, unnatural act that even some police or armed guards struggle with, as appears to be the case in Parkland, where an armed sheriff’s deputy did nothing to stop the killing. In the event an armed guard actually did intervene, more deaths or injuries would likely be the result. Armed guards exchanging fire with one or more shooters would result in a chaotic scene filled with deadly crossfire, and would complicate any law enforcement response too.

I’m a former Army officer who spent nine years in military police and civil affairs units, including a combat deployment to Iraq where I lived on a small compound in one of the most violent cities and carried loaded weapons every waking minute because of the threats around me. I am also a parent who wants to protect my kids more than anything in this world. I understand the urge to send armed veterans like me to protect our kids from the scourge of gun violence that killed more Americans last year than all the combat in Iraq and Afghanistan since 9/11. And yet, because of my training and combat experience, I know that putting armed veterans in schools will make our children and society less safe, and at tremendous cost too.