“Let others wage war. Thou, happy Austria, marry” (a description of Habsburg marriage policy)
Human inbreeding has both a genetic side (which favors outbreeding, at least within the species) and a political side which may favor a balance between outmarriage (to make new alliances), and in-marriage (to conserve old alliances, and keep land and honor within the family). The Habsburgs played the political game adroitly, putting together an enormous empire, partly by war, but partly by astute dynastic marriages. The Habsburg domains were so unwieldy that after the death of Charles V in 1558, they were divided between two branches of the family. Both sections were huge. The map below doesn’t even show the Habsburg possessions outside Europe, in Spanish America and the Far East.
By 1700, however, genetics caught up with the Habsburgs. The Spanish Habsburg line ended with Charles II, who was grossly disabled, physically and mentally. He was also impotent, and left no heirs. A recent calculation shows that, as a result of generation of in-marriage, Charles II had a coefficient of inbreeding of .254. For comparison, a child of full sibling incest will have a coefficient of inbreeding of .25!