Think of the states in Europe that were formed based around tribal identity, language or religion. These states were based on a common outward identity.  Like the present day Kurds, they wanted and needed a state in order to protect who they were and not to be overrun or subjugated by other identities or foreigners. It is very easy to see how countries separated by outward characteristics would be involved in constant war.
Our American founders recognized this in Europe and decided to build a country based not on the European model but by a unity of principles.
Consider a nation state built for the protection of the individual and his God-given or natural rights. Citizens only have to swear allegiance to an idea granting your fellow citizens the same freedoms that you claim. Citizenship requires a basic understanding and application of these Constitutional principles. This is America where nationalism unites us in principles, not race nor religion. I think Trump believes in America.