Human being have inalienable rights. Among these the basic rights are freedom of association and freedom of speech. When these freedoms are threatened it is not only moral to oppose the threat of force with force, it would be immoral not to oppose those who would deny you your freedoms.
It is also moral to defend the freedoms of other people. Not only is it moral to defend other people's freedom, it is immoral to see other people being denied their rights and to do nothing.
The nationality of the people you are defending is irrelevant. A free person's responsibilities do not end at an international border.
Is there something here I have said that is a mistake? Because when I look at (some not all) of the anti-war groups these days I see isolationist and pacifist arguments dominating and common cause being made with theocratic fascists, the worst enemies of freedom and atheism.