For a list of all Commonwealths in the Fallout universe, see Commonwealths.
The European Commonwealth is the Fallout universe's equivalent of the real-world European Union, which has integrated into a single state-like entity throughout Europe; this was probably a federal union. It is, however, unknown which European states were members.
In 2052, Europe was heavily dependent on oil imports from the petroleum-rich states of the Middle East and responded with military action to rapidly rising oil prices that were damaging its economy, starting the terrible global conflicts that later historians simply lumped together as the Resource Wars.
The outcome of the conflict suggests that the European Commonwealth possessed a unified military. It is unknown whether the Commonwealth took steps to ensure its survival like the United States did.
A prequel to the Fallout series, Fallout: Resource Wars, set in a collapsing Europe (during the European Commonwealth's invasion of the Middle East), is one of the games that J.E. Sawyer would like to make if he could do any game, regardless of money and development time.