Just a little confused as to why the practice was common enough. Would their vassals be related to them, would they commonly marry into the same family to keep an alliance going?
Like, imagine if I’m the king. Why marry my cousin when I can marry the daughter of another ruler, of another realm? Sorry if my ignorance is cringe worthy.
The TL;DR is that those foreign rulers are themselves your close relatives.
Let’s take a famous example: King Louis XIV of France (r. 1643-1715). Louis was the son of Louis XIII (of the House of Bourbon) and Anne of Austria (of the Spanish line of the House of Habsburg). So…who was Louis related to?
Taking his father’s side first, his paternal aunt was Queen Henrietta of England and Scotland, the wife of Charles I. This made Charles and Henrietta’s two sons, the future English/Scottish kings Charles II and James II, both Louis XIV’s first cousins. We aren’t done yet, though.
Louis’ maternal grandfather was King Philip III of Spain, making that king’s son Philip IV, his mother’s younger brother, his uncle. Louis’ mother Anne was ALSO linked to the Austrian Habsburgs through her mother, making them Louis’ more distant cousins.
In 1659, to celebrate the end of war between France and Spain, Louis XIV married the daughter of King Philip IV of Spain, the Princess Maria Theresa. If you’ve been following, yes she would be Louis’ first cousin, the niece of his mother. This marriage would also make Louis the brother-in-law of both the Spanish king Charles II (his wife’s brother) and Emperor Leopold I (his wife’s sister’s husband).
If all of this seems crazily complicated, remember that Louis and Leopold eventually went to war over the Spanish throne when their mutual brother-in-law Charles II of Spain died childless.