The civil war was the result of a shipwreck in 1120 when Adelin, son and heir of King Henry I of England, was drowned. That left Henry without a legitimate male heir not only for the Kingdom of England, but also for the Duchy of Normandy and County of Cotentin that he also ruled. His only legitimate child left - he had numerous illegitimate children - was his daughter Matilda. She had married Holy Roman Emperor Henry V and, after his death in 1125 she returned to England. Henry made all his nobles swear that they would accept Empress Matilda as their Queen after his death.
There was some grumbling about this act. While women could, and did, own and run great estates, it was generally believed that being a ruler was not something a woman could do adequately. Rulers were expected to lead their men into battle during wartime and to travel constantly around thier lands during peacetime to dispense justice, adjudicate disputes and get to know the knights and nobles of the land. Fighting with sword or lance demanded great physical strength, something most women did not possess, while spending long hours in the saddle was not suitable for women who spent many months either pregnant or breastfeeding. Nevertheless, allowances could be made so the nobles of England and Normany duly took the oath.
Henry then ordered Matilda to marry Geoffrey of Anjou, heir to the Count of Anjou. Matilda was outraged, believing a mere count to be beneath her imperial dignity. The barons were equally appalled, since Normandy and Anjou were traditional enemies. Meanwhile, Matilda had been making herself thoroughly unpopular with the English and Norman nobles with a tactlessness and rudeness based on her clear belief that as an Empress she was vastly superior to earls and barons. She was particularly unpopular with the noblewomen, whose fashions she denounced as dowdy and whose manners she scorned in comparison to those of the imperial court.
When King Henry died, Matilda happened to be in Anjou. Much closer to England was Stephen of Blois, Count of Boulogne. Stephen was by his mother a grandchild of King William I, as was Matilda, but he was also through his father a great great grandson of King Edmund Ironside, one of the rulers from the English dynasty that William I had destroyed at the Battle of Hastings in 1066. Not only was he of royal blood, Stephen was immensely popular and widely regarded as a talented and efficient ruler of his lands. He was also a man. As soon as he heard Henry was dead and Matilda was absent, Stephen boarded a ship and headed for England. When he arrived in London he declared that he was the true King of England. The crowds in London cheeered him loudly, and as various nobles arrived they too hailed Stephen as king. The Church too came out in favour of Stephen and a few days later Stephen was crowned king in Westminster Abbey.
from The Battle of Lincoln by Rupert Matthews.