 Following King Henry I 's death in 1135 , the succession was disputed between the king 's nephews — Stephen and his elder brother , Theobald II , Count of Champagne — and Henry 's surviving legitimate child Matilda , usually known as the Empress Matilda because of her first marriage to the Holy Roman Emperor , Henry V. King Henry 's only legitimate son , William , had died in 1120 . After Matilda was widowed in 1125 , she returned to her father , who married her to Geoffrey , Count of <unk> . All the magnates of England and Normandy were required to declare <unk> to Matilda as Henry 's heir , but after the king 's death in 1135 Stephen rushed to England and had himself crowned before either Theobald or Matilda could react . The Norman barons accepted Stephen as Duke of Normandy , and Theobald <unk> himself with his possessions in France . But Matilda was less <unk> , and secured the support of her maternal uncle , the Scottish king David I , and in 1138 also that of her half @-@ brother Robert , Earl of Gloucester , an illegitimate son of Henry I. <unk> in the Welsh Marches <unk> against Stephen in 1136 , but the revolt was not settled until 1138 . In 1139 , Matilda invaded southern England with her half @-@ brother 's support and the period of civil war began . 
