King Christian IX of Denmark reigned from 1863 to 1906 and is often referred to as the "Father-in-law of Europe" due to his children's marriages into various royal families. He was born on April 8, 1818, and became king after the death of his predecessor, King Frederick VII. His reign saw significant political changes, including the introduction of a new constitution in 1866. During his time as king, Christian IX worked to modernize Denmark and strengthen its position in Europe. He faced challenges such as the loss of Danish territories in the Second Schleswig War (1864) but maintained a focus on diplomacy and international relations. His legacy includes a more unified and modern Danish state.