There must be some. Its just that history isn't normally about not wars. I know that some wars, like the war of 1812 and both world wars, the U.S. could have entered sooner, but the U.S. eventually did. Are there any times the U.S. had a decent reason to enter war and never did? Also any notable occurrences where other countries did this?
I suppose you'd need to qualify what you mean by "decent reason".
Kennedy was certainly angry and embarrassed enough to go to war with Cuba, but didn't want to do so unless other Latin American nations gave the go-ahead. Eisenhower had, in his and the JCS's minds, a very reasonable set of reasons to go to war with China during the Formosa Crises (he also had a great deal of pressure from the military to do so), Woodrow Willson wanted to go to war with Mexico after Villa's raid on Columbus, NM, but settled for a "punitive expedition" which almost led to war. Really, almost every presidency has events which could have led to war, and certainly every single one has instances of opportunity, but eventually one has to deal with the likelihood of war actually happening.