Just look up any military tragedy, like say, the infamous My Lai Massacre, where some 500 civilians were gunned down. There were a handful of soldiers who shielded the civilians and defended them, and they were shunned and disgraced at the event and after it by the military.
It took until 2003 until these soldiers were congratulated.
These guys are trained to kill, kill, kill, and if the orders come down the pipe, like they did in My Lai people will both carry them out, and look the other way.
Soldiers are brutally conditioned to obey orders without question. When someone grows a conscience, and decides to act on their convictions they are harshly punished. Theoretically, soldiers have a duty to report unjust orders to the superiors above the person who gave the order, but I have my doubts as to how encouraged this is, much less how often it happens. Soldiers are murdering civilians overseas as we speak. Why would it be any different with us? Apparently, all it takes is an order to override the human conscience.
The U.S. Army has shown in the past of its full capability to torture to get information. In some cases, if not all, those tortured were at no point in time given any of the rights given to domestic groups. But this is besides the point. In Iran and Iraq, our soldiers intimidate, and often threaten locals. I have no doubt in my mind that the Army would obey orders.
Look up Martial Law, you might of heard of it. Simple fact is the Police and Military are the physical power behind a government without them people could just ignore the government all together.
I am not saying they are going to turn there guns on the populace without reason, but they will carry out there orders, they will maintain public order just like the police but with bigger guns and the majority of crimes will result in being either arrested or shot depending on the crime.
It would take an extreme situation for the military to act brutally towards the civilian population, but if the situation arises they will do it. And chances are they would not use local troops who may hesitate, dealing with people they know, they would bring in troops from the other side of the country if necessary to get the job done.
A soldiers job is to perform whatever acts that the people deem necessary. There loyalty to there country is not to the people but the job that they have been given. Whether they are killing people in the sands of Afghanistan or the streets if New York, they are doing there job. This is why i believe that they obey orders to act brutally towards civilians in a time of crisis.
The job of the military is to follow and protect the government. Whatever the government says is that they're obligated to do. Their jobs isn't to determine the morality of what the act is. If the government told them to kill a bunch of U.S civilians then yes they would. Some; very few may not and they will be killed and viewed as "traitors". Civilians regarding any non-militia person then yes they already have. Since the beginning of the U.S government the military has killed innocent civilians; the Indian removal, bombings of Japan, raping and killing innocents in the Vietnam war and many other times. The military is nothing more than the bronze of the government; their job is to do what the government tells them to do. Fight who the government tells them to fight and kill all who the government says to kill.
The US military is not homogeneous. There is no morality test when you join. The higher you go, the more ambitious one must be to continue advancing. Most retain their moral compass. Many, however, do not. With cases like Abu Gahrib, Haditha, and numerous extrajudicial killings emerging from our most recent wars it should not come as a shock that the military does always do the right thing or uphold the highest standards.
Bear in mind the oath of service requires a Service Member to defend the US against all enemies both foreign and domestic. The military has been used domestically to quell racial issues and restore order. If there were a genuine crisis, the military wold respond if ordered to do so. Trust me, there are many in the higher ranks who would calculate that failure to obey the direction would simply result in them being fired and replaced by someone who would.
Its why the US Constitution has established civilian control of the military. The vast majority of the Service and those who serve are honorable, but it only takes one to corrupt the entire thing.
If a rebellion were to ever start (I predict that a economic collapse would spark it all) sure you would have the occasional yahoos who will obey orders without question. However, all you have to do is look at organizations such as oath keepers and you will see a trend among us military that they may take our side once the rebellion (if it ever happens) starts.
It is organizations like these (and the fact the among the protesting crowds may be family) that will create a sort of "chivalric code" among solders that will prevent them from doing so and shame those who do the opposite.
In my prediction, overthrowing the feds would be a bit messy and slow but CAN happen (be it peacefully or not). However, its what comes after where I get a bit concerned.