Unification
I was going to start with a slogan example illustrating a point, but I realized I need to make a disclaimer, because it came out quite offensive :) The example was similarity between "one nation under one god" and "one folk, one reich". Now, I am not comparing The 3rd Reich to USA, and I do not think it is justified to draw that comparison in any case. There are very significant differences. Of course.
However... my point was that unification of people can be done either basing on some external factors, like being of the same language, color, geographical location and/or traditional notion of a "nation" - abstract idea, mythological creature, but somehow making people feel good about belonging to a group and willing to work for the group. Or willing to make sacrifices that are deemed necessary by those who happen to govern that group. It turns out groups and their symbols (flags, anthems, etc) are very attractive to people, and if in the name of the flag the people are asked to, say, go to war, it is difficult to say no, especially if the rest of the group applies pressure. That's external unification, by those who themselves not necessarily value symbols themselves, or feel any connection to the group or people who participate in the group - if the leaders were true leaders, they would lead the troops themselves. It happened in the old days. Now, is there any political leader who'd like to be in the front line? Ha. It is never them, nor their children. Nonetheless, they overuse the words like 'patriotism', or 'greatness of the nation' or something like that, so people feel good about buying tanks and sending them abroad even if there is not enough food around, or medical care, etc. External unification does not produce unity, it produces armies of obedient slaves. That's what it is for, that's what it's been for quite some time now. However, feeling good about being a member of a group does not necessarily contradict the other kind of unity, internal one.
Internal unity is knowing that abstract groups like nations, casts, etc consist of smaller groups (indefinite number, to be precise) and themselves constitute bigger even bigger groups, including but not limited to the whole of our species, then mammals, then life itself. One species under god, how about that? God is, however, not necessary here. One nation, one planet, one peace? One universe? If our Universe had a flag, what would it look like?
From that point of view going to war or supporting any kind of oppression seems ridiculous. We are all people and we live on one planet (although that is going to change soon). Nations and flags are abstract creatures - they are harmless if used properly, but they can also be really misused if a nation considers itself to be a "chosen one", with mission to lead the others. Or erase the others. Asking which nation is better is like asking which clock is fastest - it's absurd...
Feeling of unity should come from one's heart. And it's not even something you need to learn or get from somewhere - you already have it, and the trick is to remove false impressions of divisions between people, as they usually are illusory and... more often than not they have been installed on purpose. Someone out there wants us to be divided, sorted into groups, so we can be played against each other.