A great war involving most of the countries of
Western Europe, lasting from 1701 to 1714. It arose because the same person,
Philip of Anjou, looked set to become heir to both the
Spanish and the French
thrones.
This would have created a superpower: not only did both have vast overseas empires, but Spain's European possessions included half of Italy (Milan and all of the south) and half of the Netherlands (the Roman Catholic part now known as Belgium).
Spain was ruled by the senior branch of the highly inbred Habsburg dynasty: since 1665 the king had been Charles II (or Carlos II), mentally retarded and with no children. As his death approached, his closest heir was his sister's grandson. The Infanta Maria Theresa had married Louis XIV of France, and their grandson Philip therefore was in line for both thrones.
The other European powers proposed alternative candidates: first the son of the Elector of Bavaria, then on his death in 1699 the Archduke Charles of Austria, representative of the secondary branch of the Habsburg dynasty. But in 1700 King Carlos died, leaving his throne to Philip. Voltaire records that Louis XIV said Il n'y a plus de Pyrenées, "The Pyrenees are no more", disposing of the mountains between France and Spain.
A Grand Alliance was now formed to prevent Philip from enjoying his new throne. England and Scotland (united in 1707 into Great Britain), Hanover (united with Great Britain in 1714), Prussia, Holland, Austria, Savoy, Portugal, and the central authorities in the Holy Roman Empire, all made war upon the alliance of Spain, France, Bavaria, and the Palatinate (Cologne).
There were several fronts: civil war erupted in Spain; in northern Italy the Austrian commander Prince Eugene of Savoy fought against the Spanish rulers of Milan; in the south of Germany Louis of Baden fought against the Bavarians; and the maritime powers blockaded France and cut off the overseas empires.
The greatest victories (well, in my English-born curriculum) were those of the Duke of Marlborough at Blenheim in 1704, then against the French in and around the Spanish Netherlands: Ramillies in 1706, Oudenarde in 1708, and Malplaquet in 1709. The Rock of Gibraltar was also seized in 1704.
By the Treaty of Utrecht in 1713 (aided by the Treaty of Rastatt in 1714) Britain obtained territorial concessions from France in the New World (Acadia and Hudson's Bay), it was agreed that King Philip V of Spain would never become King of France (Louis XIV was succeeded in 1714 by his great-grandson), and Spain's European territories in Italy and the Netherlands were handed over to the Austrian Habsburgs.
All those date softlinks were already there on the nodeshell - don't blame me. Thanks to Gritchka for gently telling me where Blenheim was.