Flag-waving is a fallacious argument or propaganda technique used to justify an action based on the undue connection to nationalism or patriotism or benefit for an idea, group or country. It is a variant of argumentum ad populum. This fallacy appeals to emotion instead to logic of the audience aiming to manipulate them to win an argument. All ad populum fallacies are based on the presumption that the recipients already have certain beliefs, biases, and prejudices about the issue.
If flag-waving is based on connecting to some symbol of patriotism or nationalism it is a form of appeal to stirring symbols which can be based on undue connection not only to nationalism but also to some religious or cultural symbols—for example, a politician appearing on TV with children, farmer, teacher, together with the "common" man, etc.
The act of flag-waving is a superficial display of support or loyalty to, for example, a nation or a political party.
Today, indeed, flag-waving has become a quite common generic term denoting the deliberate appeal to nationalistic emotions and prejudices.
Flag-waving is a popular propaganda technique, meaning that an action is justified "on the grounds that doing [what is promoted, in this case support the war] will make one more patriotic, or in some way benefit a group, country, or idea.
...ad Hominen ridicule, ad Misehcordiam tears, or ad Populum flag-waving - all logical fallacies...
The ad populum argument presumes that the audience already holds a particular attitude and specific beliefs on the issue.