Antwerp was an Earth city in the northwestern part of Belgium, close to the Dutch border, and the capital of the eponymous local province.
In 2372, Antwerp was the location of a high level diplomatic conference held between the United Federation of Planets and Romulan Star Empire. It was bombed by a Changeling infiltrator, killing 27 people, including the Tholian observer. The attack signified a Dominion presence on Earth. (DS9: "Homefront")
It is unclear whether or not Antwerp hosted a specific Federation office, such as Paris, San Francisco, and Lisbon did.
From its proximity to Brussels, and more importantly both lying within the current nation of Belgium, it seems likely that Antwerp was part of the European Alliance.