Send in the National Guard! Many Republicans have issued that call as a leading solution to the crisis at the border. Speaker John Boehner has urged Obama to send in the Guard to provide humanitarian relief to arriving children and relieve the Border Patrol so it can focus on border security. The coming House GOP proposal on the crisis is likely to include a similar prescription.

It’s also possible Obama could agree to a role for the National Guard in the final proposal, if it’s necessary to win over Republican support.

But in an interview today, the head of the National Guard under George W. Bush said he had not yet heard a clear rationale for sending in the Guard and suggested it might not be the appropriate response to the problems at the core of the current crisis, though he did say he could envision the Guard playing some sort of part in a broader solution.

“Until mission requirements are clearly defined, it can’t be determined whether this is an appropriate use of the Guard in this particular case,” H. Steven Blum, who was the Chief of the National Guard Bureau from 2003 to 2009 and has been a career military man for decades, told me. “There may be many other organizations that might more appropriately be called upon. If you’re talking about search and rescue, maintaining the rule of law or restoring conditions back to normal after a natural disaster or a catastrophe, the Guard is superbly suited to that. I’m not so sure that what we’re dealing with in scope and causation right now would make it the ideal choice.”

The Guard was sent to borders twice during Blum’s tenure, and both raise questions about whether it is relevant to the current crisis. The first, Operation Winter Freeze, involved efforts to prevent terrorists from crossing from Canada. The second, Operation Jump Start, involved sending 6,000 National Guard members to the southern border to provide backup support for the Border Patrol, so it could do “hands-on law enforcement, and not be tied down,” Blum confirmed.

But it’s unclear whether the current crisis demands similar solutions. According to the Arizona Republic, the debacle is occurring almost entirely at a single border sector — the Rio Grande sector — calling into question claims that the crisis shows broad chaos all along the border. The causes range from violence in Central America to human traffickers exploiting mis-perceptions of U.S. law to the accurate perception that legal processes allow kids to disappear into this country while awaiting a court date. Indeed, Republicans themselves have premised their evolving legislative response largely on the idea that a chief cause is holes in the 2008 trafficking law, which is why they are calling for changes to it that would make faster removals possible. (Obama is open to similar changes.)

But if that is the cause of the problem, it’s unclear what sending in the Guard would accomplish. The call for the Guard to provide humanitarian relief is also puzzling, given that Republicans themselves are criticizing the parts of Obama’s request for funding that would provide humanitarian relief.

In fairness, when governors like Rick Perry of Texas call for the Guard to respond to the current crisis, it’s in some ways understandable. Perry and other Governors have frequently relied on the Guard to respond to crisis situations, such as disaster relief after Hurricane Katrina, so a similar call here is not that surprising, Blum notes.

But even Perry has struggled to articulate what the Guard should do in this case. When Brit Hume recently pointed out to Perry that kids are merely turning themselves over to law enforcement already, and asked how the Guard would change this, Perry argued that the “visual” showing them at the border might serve as a deterrent.

That seems of a piece with a general political argument Republicans have made: That Obama’s general failure to secure the border and project a sufficient air of border toughness are the cause of the problem. In service of this case, Republicans have blamed Obama’s failure to deport the DREAMers (Obama”amnesty”) for the problem, too.

Blum notes that one could envision the Guard playing some sort of role in a broader solution — but says he hasn’t yet heard anyone plausibly explain how it should work. “I’m not sure we’ve clearly defined the question, and until that is done I am reluctant to tell you that the Guard is the answer,” Blum says. “Merely sending the Guard to the border is not a panacea for the myriad complex problems of the current situation.”

Indeed, the onus should be on those calling for the Guard to come in to explain in more detail what it should do. Until then, it’s fair to ask whether calls to send in the Guard are as much about trying to hype up notions of widespread chaos at the border as they are about anything else.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.