If we go into Iraq without a huge coalition, it is going to be nothing but bad. It won't be a problem taking out Iraq, but we have no real plan for anything after that. Meanwhile the hatred in the Muslim world toward the western world, and the US in particular, will grow like the worst cancer. 
The question is who is not in the coalition that could hurt us. Germany? France? Russia? Who is it because these will all magically fall in once we start the war