Nationalism starts with the military, the group of people who you can count on to have your back, if we as a nation are attacked or threatened. These people don't know you, you'll likely never meet many of them, and yet they'd die for you. Why? It's not like they chose to be born American, or grow up in America, that was a decision their parents made for them, and a decision which maybe their parent's parent's made for them and so forth. That is essentially the core of nationalism: family, and bonds. It's the idea that we're in this together, for better or worse. The desire to keep the "outsider" out originates not out of hatred for those who are different but from a love for one's people, a desire to preserve it, and a desire to also preserve the cultures and people of the world.
America is unique in that we are a very young nation, and nation comprised of many different people, our culture has been overshadowed by the globalist consumer culture, our identity was really only just forming in the 1950's, and not soon after was crushed by the ideas of diversity and pluralism. Critical theory can reduce any of the great ideas, and make them sound ridiculous. I would counter, that only ridiculous people, would believe that an idea like nationalism could be sufficiently boiled down to something as ridiculous and simple as "birth by proximity". Nationalism is closely tied to history. It is an almost sacred appreciation for those who came before you, and those who will come after you. It is an appreciation for the ebb and flow of the borders of a nation and an empire, and the blood and sweat that made it possible. It is an appreciation of the necessity of both the working class and the aristocracy, of the necessity of both the man and the woman, the young and the old. It seeks not to unify or equate the poor with the rich, nor to turn them against each other, but to create harmony.
Our present society is very future-oriented, we like to think that history doesn't repeat itself. We like to think that we are the exception. That we are special. That we are making progress that has never been made before. The reality is that as Spengler and Toynbee note, that civilizations have lifespans. That the very things that allow civilizations to ascend to great heights also often hasten their demise. Gay marriage is nothing new, during the decline of the Romans it was common place. Universal suffrage and racial equality, despite our pride in the strides we've made, existed in the ancient era. This is the consequence of looking ever forward, is that you lose the sight of the past. There are generations today who feel entitled to healthcare (ours being among the best in the world), and it wasn't even two generations ago that even having an ice box was a luxury, water had to be pumped and heated. Not long ago electricity wasn't even a reality for most people. If a rogue locust swarm wiped out your crops, you didn't have a government check you could count on.
Progress is unfolding yes, and the future will never cease to amaze us. But we lose part of who we are as humans, when we forget how simple life can be. I'm not saying that we should live like the Amish, but I am saying that a tangible and gripping understanding of that kind of life will not only lend us greater appreciation for our current way of life, but also guide us away from what is superfluous and towards what is meaningful.
The nationalist doesn't have to defend his beliefs, because they are self evident. All great nations and empires were born by these principles, and all great nations and empires passed on into history having forgotten them.