The United States worked when there was a unifying identity. This identity was powerful because it was highly visible in the culture. Being American was something to be proud of, and there was the powerful narrative of the rags to riches journey. Americans saw their country as being uniquely great.

Research shows that people of disparate backgrounds (e.g. ethnicity, class, gender) can achieve cohesion through a common identity or goal (Allport and Pettigrew are good sources on this). As the US has become more balkanised, largely through the efforts of neo-Marxists, this common identity is diminishing and people are retreating in to the tribes with which they identify. This never ends well.

Diversity, at least as identitarians understand it, is not a strength. It is historically the source of war and civil unrest. There is a massive cost to the neo-Marxist strategy of 'deconstructing' Western cultures. They do this under the premise of attacking systemic racism and nationalism. While nationalism can have a dark side, when it is directed to a particularly negative view of the out-group (other nationalities), it is what binds a country together.

All of this is easily demonstrated. What happens in a football club when it begins admitting people who don't like football or prefer to play other games? The character of the club and its area of focus changes. Instead of a group of people with a shared interest in football you have separate groups all doing their own thing. These groups will interact only where it is necessary, but will prefer to spend most of their time within their respective subgroup that caters to their interests. Where the groups are competing for resources (e.g. use of the pitch), conflicts will arise due to competing interests. The majority will feel alienated within their own club, leading to the rise of extreme behaviours aimed at restoring the club to its original purpose. This very pattern is currently being played out at the national level.