Michaelmas he appears to have been sent to the Count of Flanders with money and troops. After taking a prominent part in the preparations for war in 1214 he was made marshal of the king's army in Flanders, and joined forces with the emperor Otto IV and the other allies of John against Philip of France. On 27 July he commanded the right wing of the allied army with the Counts of Flanders and Boulogne. He was taken prisoner and was given by Philip to his kinsman, Robert, count of Dreux, in order that the count might exchange him for his own son Robert, who had been taken by John shortly before (Wendover, iii. 287 sq.; Gulielmus Armoricus, sub an.). The exchange could not be effected immediately (Fœdera, i. 124). On his return to England he held aloof from the confederation of the barons, though after they entered London he was forced to assent to their proceedings. He stood among the king's friends at Runnymede in June 1215, when John granted the Great Charter, in which his name appears as one of those who counselled the grant. In December John made him one of the captains of his army in the south, and he took measures with Falkes de Breauté [q. v.] to have London closely watched in order to cut off the supplies of the baronial party while he and his fellow-captains overran Essex, Hertfordshire, Middlesex, Cambridgeshire, and Huntingdonshire (Wendover, iii. 347). He also took part with Falkes in cruelly ravaging the Isle of Ely in the early weeks of 1216, joined the king, helped him at the siege of Colchester, and on John's behalf swore to the terms on which the place was surrendered by the French allies of the barons (Coggeshall, p. 179). He remained faithful to the king until after the middle of the year, but Louis having landed and taken Winchester on 14 June, Salisbury, no doubt thinking that the king's cause was hopeless, joined Louis, and yielded to him his castle of Salisbury (ib. p. 182; Chron. de Mailros, p. 191; Stubbs, Constitutional History, ii. 15).
After the death of John, Salisbury was sent by Louis to Dover to persuade Hubert de Burgh to surrender the castle, and was severely reproved by Hubert for acting against his own nephew, the young King Henry III. By December he showed an inclination to desert the French side, attended the council of Henry's supporters at Oxford in January 1217, and on the departure of Louis joined the king's party, covering, in common with other lords, his political change by taking the cross, and professing to engage in the war at the bidding of the legate as a crusader (Walter of Coventry, ii. 235). He received the restitution of his estates and the sheriffdom of Somerset in March, fought in the royal army at the battle of Lincoln on 19 May, and was appointed sheriff of the county. In August he took part in the naval victory of Hubert de Burgh [q. v.]. He affixed his seal to the treaty of Lambeth with Louis on 11 Sept., acting then and in 1218 as one of the council (Fœdera, i. 148, 152). Having been in alliance with William of Aumâle he wrote, perhaps towards the end of that year, to Hubert de Burgh informing him that the alliance was at an end and that he was not to be held responsible for any ill-doings of the earl (Royal Letters, i. 19). It is asserted by Matthew Paris (iii. 49 n.) that he joined the crusading army at the siege of Damietta in 1219, and in August greatly distinguished himself by his gallantry. It is, however, exceedingly doubtful whether this was so. Paris variously describes the crusader as ‘comes de Salebregge,’ ‘de Sarreburge,’ ‘Sausbrigiæ,’ and ‘de Saleberge,’ and appears to couple him with the Earl of Chester [see Blundevill, Randulph de]. The chronicler appears to depend on the somewhat late authority of ‘L'Estoire de Eracles,’ lib. 32, cap. 12, p. 343 (Dr. Rohrichte, Publications de la Société de l'Orient Latin, ii., comp. p. 51 and Index), and on passages in Wendover, iv. 54, who copied from Oliver Schol. p. 1139, or Jacques de Vitry, p. 1101. A like statement figures in the ‘Gesta Crucigerorum Rhenanorum,’ p. 51; but there the crusader identified with Salisbury may as well be coupled with the Counts of Holland and Weid as with the Earl of Chester (see also Bernard Thesaurar. c. 198). There is negative evidence against the presence of the earl at Damietta, specially the difficulty of fixing a date 1218–1220 when he could have been absent from England for any length of time (see Calendar of Close Rolls, pp. 360–406). It may, therefore, probably be inferred that the crusader was not the Earl of Salisbury, but was the Count of Saarbrücken. Joinville, in his ‘Mémoires,’ c. 59, calls the Count of Saarbrücken, his companion on the crusade of 1249, ‘le Conte de Salebruche’ (information supplied by Mr. T. A. Archer).
On 28 April 1220 the legate Pandulf laid two of the foundation-stones of the new cathedral of Salisbury on behalf of the earl and of his countess. In May the earl wrote to Hubert de Burgh against the proposed appointment of William of Aumâle as seneschal of Poitou, and about June to inform him that he had been sick, but hoped to be able to attend the conference to be held at York (ib. i. 129, 136). At the excommunication of William of Aumâle at St. Paul's in January 1221, Salisbury appeared