I believe the arguments against using the Military along the border to assist Homeland Security officers may be valid and surely deserve consideration depending on their reasoning for their objections. The whole thing, I believe, centers around the Posse Comitatus act and interpretation of whether Trump or any military officer who obeys Trump's order, is in violation of that law/act.
There are those that object to Trump and everything or anything he does or decides to do but in this case, using the military for such a cause could be construed as a violation of the act. That is an answer that Courts will have to decide but is a danger that Trump could be considered as overstepping his authority and misusing the military depending on what orders he is issuing to them.
The real question is whether or not those coming are a National Threat or whether or not this is just a police action to enforce existing laws. If an invasion and a National Threat then President Trump may have cause to use the Military but given the numbers and cause expressed by the bulk of the people in the group the Military may have good reason to reject such a command under the posse comitatus act. I have fully supported Trump but in this case he needs to be very cautious and careful not to violate the act for he doesn't need to give his political enemies any credible charges to hold against him. I do believe in this case, given the people involved, Trump needs to exercise restraint and caution because he could put himself in violations of the act.
I do realize Wikipedia is not often totally accurate and can be amended by almost anyone but still the link above gives a pretty clear picture of why I think Trump should be cautious and why or how many Military officers or people might honestly refuse to obey Trump's order if their interpretation is that it is a violation of the act or would be.
