Although Spain had taken the lead in the exploration of the northwest Pacific Coast, with the voyages of Juan José Pérez Hernández in 1774 and 1775, by the time the Spanish determined to build a fort on Vancouver Island, the British navigator James Cook had himself visited Nootka Sound and charted the coast as far as Alaska, while British and American traders had begun settling on the coast so as to develop resources for trade with Europe and Asia. In 1793 Alexander Mackenzie. a Scottish born Canadian working for the North West Company crossed the continent and with his aboriginal guides, French-Canadian voyageurs and another Scot, reached the mouth of the Bella Coola River, completing the first continental crossing of North America north of Mexico, missing George Vancouver's charting expedition to the region by only a few weeks. The competing imperial claims between Russia, Spain and Britain were compounded by treaties between the former two powers and the United States, which pressed for the annexation of most of what is now British Columbia.