The League of Ostend (or Ostend Alliance) was an alliance established in 1633 in response to the arrival of the time-displaced town of Grantville, and the alliance the town established with Gustavus Adolphus of Sweden. It was established through the machinations of Cardinal Richelieu, and included the countries of France, Spain, England, and Denmark. The League's primary goal was to capture the Baltic Sea, crush the independent Netherlands, and, eventually, eliminate the growing power of the United States of Europe. Luckily, the Alliance's initial attacks were thwarted at Luebeck and Wismar, in no small measure through the impact of American technology, hastily adapted for war.
Richelieu's actions were based on the knowledge he gained from Grantville's history books. He reversed his OTL decades-long policy of using French power to thwart the Habsburg dynasty controlling Spain, and the regional power of the Habsburg Emperor Ferdinand II. Previously, Richelieu covertly blocked Spanish ambitions to regain the Low Countries and had simultaneously, given token financial support to Gustavus Adolphus' intervention on behalf of the Protestant princes of the Germanies against the German Catholic League.
League of Ostend member states included Protestant kingdoms Denmark and England as well as Catholic France and Spain. The league's active manifestation (suspected by Rebecca Stearns after her audience with Richelieu in 1633) began with a treacherous Pearl Harbor-like event. The Protestant Dutch, recently independent of Spain and the Spanish Netherlands, sailed with the allied fleets of England and France to oppose a new invasion by Spain (a second Spanish Armada, in effect, complete with a large auxiliary fleet of transports carrying an expeditionary army under the command of Cardinal-Infante Ferdinand). In reality, both the French and English naval officers had sealed orders (an idea Richelieu had taken from the fiction of C.S. Forester's Horatio Hornblower series) to be opened before joining battle—orders which announced the new alliance, and ordered them to fall on the Dutch and support the Spanish fleet. This started the so-called Ostend War.
The results were devastating for the navy of the Dutch republic, and using subterfuge, the Spanish forces were able to land behind key defenses of Flushing in Zeeland and thrust their expeditionary force into the heart of the republic. Spain's forces conquered most of the Netherlands and began a siege of Amsterdam, which along with two provinces and the rump remains of a third were the only territories that did not fall into Catholic hands. The next weeks saw the League's fleets begin the Siege of Luebeck and the doubly-pyrrhic Battle of Wismar — which awoke nationalist feelings in the German population. However, despite these early advances, by 1634, the League was on shaky ground. England's support was minimal at best. Denmark was defeated and brought into a renewed Union of Kalmar in a separate peace. The defeated French Army was in disarray, and civil war seemed to be brewing in France. The formation of the Kingdom of the Netherlands sundered Spain's influence in Northern Europe.