The European Union should become a United States of Europe

The USE would benefit all its participants culturally.
The EU is the most successful peace project in history. It ended century long rivalry and warmongering. Formalizing the bond between former enemies will bring lasting peace to the European continent.medium.com
The USE would take its place as a global player on the international scene.
The USE would strengthen the coherence of the Western alliance (USA & USE) which could be vital if China becomes powerfull enough to dominate each of them separately. With the USE in place the Western alliance would not break up just because one European state would want to leave.
The USE would create a European identity. It would create a sense of belonging to, and sharing of, the European continent and its values, and create mutual responsibility for the future of Europe.
The USE is beneficial for all Member States as many strategic interests and social values in Europe are shared among all countries.
The USE would govern better than the EU.
The USE would benefit all its participant economically.

In the USE, participating countries would partially abandon their sovereignty and give it to the newly created supra-national entity. This is a serious obstacle to realising the USE.
The USE could mean that culture uniformity will occur and the richness of culture that Europe currently possesses will be lost.
Europeans do not want a unified EU.
The USE would not necessarily be a better democracy than the EU. The EU has a democratic deficit, which the USE could inherit.
As a grouping of smaller nations, the USE may not be a durable construction.
With more power and budget in play in the USE, the temptation of corruption for politicians and civil servants could be higher than it is today in the EU. Refer to the EU corruption report for more information.
By shifting political power from a national to a supranational level, the EU, as a new form of political organization, challenges the modern nation-state. Making it a classic state itself would undermine that very idea.
There are irreconcilable differences between EU member states that make the USE a nearly impossible endeavour.
This level of integration may lead to a weakening of NATO as Europe and the US will be on equal footing. NATO should not be weakened, it forms an integral part of the security system of Europe.