The E.U. was last expanded in 2013. It accepted Croatia as a neighbor. Last expanded in 2013 when it welcomed Croatia, a country that was a close neighbor of the six hopefuls. Since then, a cascade of crises — over the economy, migration, Brexit and the coronavirus pandemic — has pushed enlargement down the bloc’s to-do list. The prospect of adding new members raises questions about the unity of the bloc, its strategic goals, and the way it views itself on the international stage.