The king, indignant that an attempt should be made to exempt the vast ecclesiastical lands from taxation at a time of national crisis, sequestrated the estates of the see of Canterbury, and copied John’s conduct in 1208 by outlawing the whole body of the clergy. Winchelsea in return excommunicated all those who refused to recognize the authority of the pope’s bull... Harassed by these domestic troubles, the king could not carry out his intention of sailing for Flanders in the spring, and spent the greater part of the campaigning season in wrangles with his subjects. He was obliged to come to a compromise. If the clergy would give him a voluntary gift, which was in no way to be considered a tax, he agreed to inlaw them.
Source: wiktionary