A long-simmering dispute between Canada and the U.S. involves the issue of Canadian sovereignty over the Northwest Passage (the sea passages in the Arctic). Canada's assertion that the Northwest Passage represents internal (territorial) waters has been challenged by other countries, especially the U.S., which argue that these waters constitute an international strait (international waters). Canadians were incensed when Americans drove the reinforced oil tanker Manhattan through the Northwest Passage in 1969, followed by the icebreaker Polar Sea in 1985, both without asking for Canadian permission. In 1970, the Canadian government enacted the Arctic Waters Pollution Prevention Act, which asserts Canadian regulatory control over pollution within a zone. In response, the Americans in 1970 stated, "We cannot accept the assertion of a Canadian claim that the Arctic waters are internal waters of Canada... Such acceptance would jeopardize the freedom of navigation essential for United States naval activities worldwide." A compromise was reached in 1988, by an agreement on "Arctic Cooperation," which pledges that voyages of American icebreakers "will be undertaken with the consent of the Government of Canada." However the agreement did not alter either country's basic legal position. Essentially, the Americans agreed to ask for the consent of the Government of Canada without conceding that they were obliged to. In January 2006, David Wilkins, the American ambassador to Canada, said his government opposes Stephen Harper's proposed plan to deploy military icebreakers in the Arctic to detect interlopers and assert Canadian sovereignty over those waters.