The Pentagon rejected the DHS request for troops to serve as emergency law enforcement on the border. As pointed out here, the president could have simply used the National Guard. The use of troops was to impress his base, to broadcast to that base just how serious he was, and to make the "invasion" seem more dire then it is. All of his suggestions in recent days, including thinking he can overturn a Constitutional amendment via executive order, have really been bluster on his part, all about the midterms. It might help his cause. The most cynical president in our history, but there are suckers out there who will buy into all this BS.
https://amp.cnn.com/cnn/2018/11/02/politics/white-house-pentagon-troops-border/index.html
Washington (CNN)When the Trump administration first asked the Pentagon to send troops to the southern border, they wanted them to perform emergency law enforcement functions, CNN has learned.
The Pentagon said no.
According to two defense official familiar with the request, the Department of Homeland Security asked that the Pentagon provide a reserve force that could be called upon to provide "crowd and traffic control" and safeguard Customs and Border Protection personnel at the border to counter a group of Central American migrants walking to the US border to request asylum...
...The request was turned down because the Department of Defense felt that active duty troops do not have the authority to conduct that type of mission unless they are granted additional authorities by the President.
.....Active duty US troops are barred from domestic law enforcement unless there is an emergency, but President Donald Trump has repeatedly raised the prospect of having troops enforce the border as he
campaigns hard on the dangers of immigration in the final days before the midterm elections.
.....The Posse Comitatus law forbids the US military from enforcing domestic laws, unless there's no other choice. Military analysts say Trump can easily use the National Guard, US Marshals or personnel from Immigration and Customs Enforcement to back up border officers if need be.