There is no better defense against allegations of ""U.S. imperialism"" than the history of our uniquely amicable relations with Canada. The longest undefended border in the world--in the world's history--separates the two countries, and in crossing it, one scarcely knows it exists. Yet is the relationship as mutually satisfactory as it seems? No, say these eight authorities, and they should know if anyone does. Mr. Merchant has twice been Ambassador to Canada. The other American contributors are Dean Acheson, James Reston, and Ivan White, a veteran Foreign Service Officer. The Canadians include a historian, two newspaper editors, and a military leader; and the fact that none of their names would be recognized by the average U.S. citizen is subtle proof of at least one large reason for uneasiness about Canadian-American affairs. Our northern neighbors have cause for fear and anger: outnumbered ten to one, with over half of their industry and nearly all their national defenses controlled by us, and with their cultural and political efforts so often completely obscured in the shadow of ours, it's little wonder they worry about their autonomy. While it would be ""an exaggeration"" to call relations ""critical,"" concludes Mr. Merchant, to say ""they are uneasy and have been so for a number of years"" would not be one. Canada's position as a ""middle power"" in world politics cannot help but give her a different perspective on the present and future, and we would do well not only to recognize the differences, but to try to respect and understand them.