Development of the city stagnated after its capture by the Teutonic Knights. Initially the new rulers tried to reduce the economic significance of Danzig by abolishing the local government and the privileges of the merchants. This was exemplified by the fact that the city council, including Arnold Hecht and Conrad Letzkau, was removed and beheaded in 1411. Later the Knights were forced to accept the fact that city defended its independence and was the largest and most important seaport of the region after overtaking Elbing. Subsequently, it flourished, benefiting from major investment and economic prosperity in the Monastic state and Poland, which stimulated trade along the Vistula. The city had become a full member of the merchant association, the Hanseatic League by 1361, but its merchants remained resentful at the barriers to trade up the Vistula river with Poland, along with the lack of political rights in a state ruled in the interest of the Order's religiously motivated knight-monks. As a result, the city became a co-founder of the Prussian Confederation which formally petitioned Casimir IV Jagiellon, to incorporate Prussia, including Danzig, into the Kingdom of Poland in 1454.