The Estates elected Charles X Gustav as their new King and his short reign is best characterized by foreign wars: first a lengthy campaign within Poland and then with Denmark. In the latter case, the risky 1658 March across the Belts which resulted in the Treaty of Roskilde, would prove to be the largest permanent territorial gain Sweden ever had: Skåne, Blekinge and Bohuslän now became Swedish provinces and have remained so ever since. Charles X Gustav was not satisfied, as he wanted to crush Denmark once and for all, but the 1659 Assault on Copenhagen did not prove successful for the Swedes, much of it due to the Dutch naval intervention to the aid of the Danes.