In 1638, after King Charles I had attempted to impose an Episcopalian version of the Book of Common Prayer upon the reluctant Scots, resistance spread throughout the country, eventually culminating in the Bishops' Wars. Montrose joined the party of resistance, and was for some time one of its most energetic champions. He had nothing puritanical in his nature, but he shared in the ill-feeling aroused by the political authority King Charles had given to the bishops. He signed the National Covenant, and was part of Alexander Leslie's army sent to suppress the opposition which arose around Aberdeen and in the country of the Gordons. Though often cited as commander of the expedition, the Aberdeen Council letter books are explicit that the troops entered Aberdeen "under the conduct of General Leslie" who remained in charge in the city until 12 April. Three times Montrose entered Aberdeen. On the second occasion, the leader of the Gordons, the Marquess of Huntly entered the city under a pass of safe conduct but ended up accompanying Montrose to Edinburgh, with his supporters saying as a prisoner and in breach of the pass, but Cowan is clear Huntly chose to go voluntarily, rather than as prisoner, noting "by giving out he had been forced to accompany Montrose he was neatly easing his own predicament and at the same time sparing Montrose a great deal of embarrassment". Spalding also supports that Huntly went voluntarily. Montrose was a leader of the delegation who subsequently met at Muchalls Castle to parley regarding the 1638 confrontation with the Bishop of Aberdeen. With the Earl Marischal he led a force of 9000 men across the Causey Mounth through the Portlethen Moss to attack Royalists at the Battle of the Brig of Dee. These events played a part in Charles I's decision to grant major concessions to the Covenanters.