Looking through a collection of historical images at reddit, this one jumped out from the list (which is heavy on World War II photos). These are the nine monarchs of Europe who attended the funeral of the United Kingdom’s King Edward VII on May 20, 1910. They are:
Standing, from left to right: King Haakon VII of Norway, Tsar Ferdinand of Bulgaria, King Manuel II of Portugal, Kaiser Wilhelm II of the German Empire, King George I of Greece and King Albert I of Belgium. Seated, from left to right: King Alfonso XIII of Spain, King-Emperor George V of the United Kingdom and King Frederick VIII of Denmark.
They were all related to Edward VII; George V was his son and the rest were in-laws or cousins. The funeral was also attended by scores of queens, princes, princesses, and other royalty. U.S. president Theodore Roosevelt was there representing the United States. See the photo full-size.