Who can declare a national emergency and send troops to war?

It is the presidents primary job as head of the US military to do these things. The office was created so that we as a country can make a swift response to such emergencies. The president has a limited budget in order to fund such actions and executive orders. It is the responsibility of congress then to decide weather or not to continue to back the president lead and officially "Declare war" or to recall the troops. The president can not use the military forces on US soil against Americans. In New Orleans, the military was used to assist in aid and rescue efforts only and could not be used as a force to restore order even to the point of having to retreat when fired on by civilians and let the state militia and local police deal with those situations.