When World War I broke out, Queen Victoria’s grandchildren served as (or had married) the kings or emperors of the UK, German, Norwegian, Romanian, Russian, Greek, Swedish and Spanish governments:

King George V of the United Kingdom (Victoria’s grandson via Albert Edward/Edward VII)
Kaiser Wilhelm II of Germany (Victoria’s grandson via Vicky)
King Haakon VII of Norway (husband of Victoria’s granddaughter Maud)
Ferdinand I of Romania (husband of Victoria’s granddaughter Marie)
Tsar Nicholas II of Russia (husband of Victoria’s granddaughter Alexandra)
King Constantine I of Greece (husband of Victoria’s granddaughter Sophia)
Crown Prince Gustaf Adolf of Sweden (husband of Victoria’s granddaughter Margaret)
King Alfonso XIII of Spain (husband of Victoria’s granddaughter Victoria Eugenie)