William was sent to Poitou to command a small force of knights alongside John's bastard son, Geoffrey, in 1205 after John's plans for a full scale expedition were squashed.

Salisbury was a commander in the king's Welsh and Irish expeditions of 1210–1212 and was appointed Viceroy of Ireland, jointly with John de Gray, Bishop of Norwich, when the king left for England in 1210.
The king also granted him the honour of Eye in Suffolk.

In 1213, Salisbury led a large fleet to Flanders, where he seized or destroyed a good part of a French invasion fleet anchored at or near Damme.
This ended the invasion threat but not the conflicts between England and France.
In 1214, Salisbury was sent to help Otto IV of Germany, an English ally, who was invading France.
Salisbury commanded the right wing of the army at their disastrous defeat in that year at the Battle of Bouvines, where he was captured.

By the time he returned to England, revolt was brewing amongst the barons.
Salisbury was one of the few who remained loyal to John, except for a few months in 1216.
In the civil war that took place the year after the signing of the Magna Carta, Salisbury was one of the leaders of the king's army in the south.
He was made High Sheriff of Wiltshire again, this time for life.
After raising the siege of Lincoln with William Marshall he was also appointed High Sheriff of Lincolnshire (in addition to his current post as High Sheriff of Somerset) and governor of Lincoln castle.
However, after the French prince Louis (later Louis VIII) landed as an ally of the rebels, Salisbury went over to his side.
Presumably, he thought John's cause was lost.

After John's death and the departure of Louis, Salisbury, along with many other barons, joined the cause of John's young son, now Henry III of England.
He held an influential place in the government during the king's minority and fought in Gascony to help secure the remaining part of the English continental possessions.
He was appointed High Sheriff of Devon in 1217 and High Sheriff of Staffordshire and Shropshire in 1224.
Salisbury's ship was nearly lost in a storm while returning to England in 1225, and he spent some months in refuge at a monastery on the French island of Ré.
William was sent to Poitou to command a small force of knights alongside John's bastard son, Geoffrey, in 1205 after John's plans for a full scale expedition were squashed.
Salisbury was a commander in the king's Welsh and Irish expeditions of 1210–1212 and was appointed Viceroy of Ireland, jointly with John de Gray, Bishop of Norwich, when the king left for England in 1210. The king also granted him the honour of Eye in Suffolk.
In 1213, Salisbury led a large fleet to Flanders, where he seized or destroyed a good part of a French invasion fleet anchored at or near Damme. This ended the invasion threat but not the conflicts between England and France. In 1214, Salisbury was sent to help Otto IV of Germany, an English ally, who was invading France. Salisbury commanded the right wing of the army at their disastrous defeat in that year at the Battle of Bouvines, where he was captured.
By the time he returned to England, revolt was brewing amongst the barons. Salisbury was one of the few who remained loyal to John, except for a few months in 1216. In the civil war that took place the year after the signing of the Magna Carta, Salisbury was one of the leaders of the king's army in the south. He was made High Sheriff of Wiltshire again, this time for life. After raising the siege of Lincoln with William Marshall he was also appointed High Sheriff of Lincolnshire (in addition to his current post as High Sheriff of Somerset) and governor of Lincoln castle. However, after the French prince Louis (later Louis VIII) landed as an ally of the rebels, Salisbury went over to his side. Presumably, he thought John's cause was lost.
After John's death and the departure of Louis, Salisbury, along with many other barons, joined the cause of John's young son, now Henry III of England. He held an influential place in the government during the king's minority and fought in Gascony to help secure the remaining part of the English continental possessions. He was appointed High Sheriff of Devon in 1217 and High Sheriff of Staffordshire and Shropshire in 1224. Salisbury's ship was nearly lost in a storm while returning to England in 1225, and he spent some months in refuge at a monastery on the French island of Ré.
William Longespée, 3rd Earl of Salisbury