I'm curious to know if royal families, while separated by countries, knew each other on personal or intimate levels. Are there any good stories of royal children growing up together and, as adults, having to deal with each other on significant political matters?
Yes! A lot of royal families were related through some bloodline. I think one of the most famous examples is Queen Victoria who was nicknamed "the grandmother of europe" who with her spouse Albert of Saxe-Coburg and Gotha had nine children that married into royal and noble families across the continent.Some examples of their descendants include:
Knowing this makes world war one something truly bizarre. King George V of Great Britain, the German Kaiser Wilhelm II were first cousins. Supposedly the Kaiser once said that if their grandmother Queen Victoria had still be alive she would never have allowed them to go to war with each other.
I would like to type a whole lot more about it here but most of what I know about this is not from books I have read in the past and not currently in my possession so sourcing would be a bit of a problem. I do have some sourcing for the parts I have told though:
edit: should check my own sources before typing. The russian Tsar was not a first cousin of King George and Kaiser Wilhelm as I originally stated.
There is a documentary that looks at the children of Christian IX of Denmark who is called the Father-In-Law of Europe. His children (and their spouses) are as follows:
Frederick VIII of Denmark married Princess Louise of Sweden
Alexandra married King Edward VII (great grandfather of Queen Elizabeth II)
George married Olga Constantinovna of Russia and was King of Greece (and grandfather to Prince Phillip)
Dagmar married Alexander Alexandrovich Romanov (Alexander III of Russia)
Thyra married Ernest Augustus who was the exiled heir to the Kingdom of Hanover.
Prince Valdemar married Princess Marie of Orléans.
The families of many of the royal houses were intertwined and have been for centuries as royalty really only married royalty.
I'm not sure what time period you have in mind, but:
If you look at Queen Victoria's family tree, you can see that while she was monarch of the UK and all its colonial territories, she married her German first cousin Albert and her children went on to marry royalty from all over Europe. Her granddaughter Alix, who later changed her name to Alexandra, was married to Nicholas II of Russia and was deposed with him during the March 1917 Russian Revolution (both of whom were later executed in 1918 with their children). To get an idea of how important Albert's family was to European royalty, take a look at his own family tree.
Another quick example: in the 12th century, Eleanor of Aquitaine was married to Louis VII of France for 15 years before she married Henry II of England. Just looking at her children's titles, they scattered pretty widely over Europe (Champange, England, Saxony, Brittany, Castile, Sicily).
William of Orange grew up in Flanders at the court of Charles V in Gent. Later Charles´ son, Philip II, became his bitter enemy on the battlefield during the 80 Years War. They knew each other quit well..
There are numerous examples of royal families going to war. Especially during WW1: The Russian Tsar Nicholas II and the German Kaiser Wilhelm II were related, just as they were related to the British Royal family.
Many royal families were related by blood.
For example, the Dutch 'Van Oranje-Nassau' were directly related to German royalty (most notably the Habsburgs), and to this day the Dutch and English royal families have narrow ties...at one point a Dutch royal was even offered the British throne!
