That really depends, does the army respond well during an unwarned disaster that struck half of the country? The answer is probably no. Also, even if there is a proper warning beforehand, there might be the issue of morale in the troops with those that are affected be less likely to function well as a soldier, which in turns mean they dead easy and adds to the undead number. Then there is logistics, is the nation ready to have fallout shelters to house most of its citizens? Food and water? Security, most likely during an anarchy. For the record, zombie outbreak shows the good and bad of people, case and point the real life examples of passengers who got off the cruise and those who self quarantined are now treated like pests. So we may as well just assumes the worst during the Z outbreak, how will the army deal with such issues? Too light and nothing is done, too hard however and a lashback will occur resulting in a vicious cycle of violence.