It is well known that European royal families tend to marry among each other to either maintain or expand their power-base. Yet the Danish King Christian IX managed to take it to a whole new level. Disregarding a lousy start of his reign, namely losing a war and nearly half of Denmark's territory, the marriage politics of his children made up for it all.
His children married into incredibly powerful royal houses in Europe, and often times had children of their own that in their turn too married into prominent royalty. Because of his cunning marriage politics, King Christian IX of Denmark is remembered as the ‘father in law of Europe’, and rightfully so, might I add.