Nationalism is all around you, whether it be the pick-up truck with the American flag or the masses standing for the National Anthem. Or whether it be any other practice or patriotic gesture you may see. It is seemingly everywhere. It is a disease that is spread by people in power. Hoping to spread among the masses to divide people and make them sympathize less with other humans. Not because these people are bad or even evil, but simply because they are not from a specific geographical location.
From this Nationalism much immorality is driven into the minds of humans. Which cause individuals to believe rights come from government, a specific government, and that flag and song represent those rights. However only those under authority of that specific government has those rights, and all others will be easily disregarded as lesser or illegal humans. Nationalism is a part of a desensitization tactic, where ones gives up the ability to think right vs wrong, based on the fact ones feels better or superior to others. This is due to the pride people have in which flag they wave.
It is not a secret to people that we are all human or that we are more similar than different. In reality though, how well would that really go over? Promotion of loving ones brothers and sisters all the same would not fair well if a government were to bomb those same brothers and sisters. This is why it is important to spread the social disease of Nationalism, for once you do that, you can commit horrors in the name of your country and your "people". To be a Nationalist is to have false pride in a geographic region of the world and people in it, a false pride that people are for any reason better and entitled to superior rights than another.
A Nation can be much wealthier and even have much better standards of living, but to believe those people have rights others do not, to believe those people are specially granted authority by government is delusional. It is the exact response a government could hope of its people, for them to bear emotional attachment to country and cloth.
Once you are emotionally attached and feel good about this flag and spend time with others reflecting on it, you will be hard pressed to give up those feel goods. You will with emotion resist this truth that you are infected with the disease of Nationalism. Symptoms include: Border policy, Flag waving, National Anthem standing, immigrant shaming, authority appealing, and just for fun feel free to add symptoms of this disease into the comments.