Had Britain not given a war guarantee to Poland in March 1939, then declared war on September 3, bringing in South Africa, Canada, Australia, India, New Zealand, and the United States, a German-Polish war might never have become a seix-year world war in which fifty million would perish.
That's Pat Buchanan in Churchill, HItler, and the Unnecessary War: How Britain Lost Its Empire and the West Lost the World. The basic argument seems to be that Britain and France could have (and should have) employed a kind of policy of "dual containment" vis-a-vis Hitler and Stalin. I don't think I share Buchanan's rosy assessment of Hitler's intentions. I probably won't finish the book, but anyone interested in the conservative anti-imperialist tradition may be interested to know that people do really believe this stuff.