Being in the military and indicating you will not follow orders goes a bit beyond expressing an opinion. I don't think there is a law that defines where the line is, but common sense suggest that you should not voluntarily join the military service unless you are willing to support your country's foreign policy.
You might try to argue that Iraq is a special case, but you would be wrong. Haiti and Kosovo were undertaken without U.N. approval and without any imminent danger to the nation. The big difference is that Republicans, even if they disagree with a democrat's policies, seldom undermine those policies in time of war. Hence you will find no instance of George Bush criticising the bombing of Kosovo, even though the results have been disasterous for the people of the region.
International A.N.S.W.E.R., a far-leftist group, has disdain for the United States. It considers our military an occupying and colonial power. It wants us out of Iraq, Haiti, Korea, Afghanistan, the Phillipines, Colombia, Cuba, and Venezuela. It demonstrates against our country in the streets while we are at war. It has proudly displayed a banner saying WE SUPPORT OUR TROOPS WHEN THEY SHOOT THEIR OFFICERS. A man who is active duty military publicy endorses their views and their march. Yes or no. If your life depended on the man sharing your foxhole, would you want someone like Jameel sharing that foxhole?