The United States never really has had much reason to declare war on anyone. Mexico was in the middle of a violent civil war and has no ability to fight the United States. Germany's navy was bottled up in the Atlantic by the British and only their U-boats had any remote freedom to move around. The British lied about the Lusitania being neutral when it was smuggling weapons to Europe. America had no reason to feel threatened by anyone in The Great War.
There was no justification for the United States to fight Germany and Italy during World War II. The only reason this happened was because Hitler declare war on the USA two days after the bombing of Pearl Harbor. If the United States had minded its own business and just let Japan colonize western China by not sending volunteers to fight them in the air and cutting off their oil supply, there was not much incentive to attack the Allies. I think the only motivation Japan had for pushing the British, French, Dutch, and American colonial possessions out of the Pacific in December 1941 was its own paranoia they were going to be attacked soon. It might have also been because they saw how poorly Britain and France had fought against the Germans in Europe and took advantage of their weakness to annex their territories, thinking America would be demoralized and not fight back either because it was a peacetime nation and still suffering from the Great Depression.
Japan was no really military threat to the United States because it had no capability of attacking the American mainland and could fight on the sea or invade its territorial possessions. I say it would have made no difference if the Japanese had taken Guam, Midway, the Phillipines, and others because they were conquered during the immoral Spanish-American War and not maintained. Imagine if the Spanish War had never happened in 1898, Spain would have still been in charge of those territories in the Pacific and under Francisco Franco been a fascist ally of the Japanese Empire. If American had not annexed the unneeded Hawaiian Islands, the British most likely would have acquired them and it would have been of no concern to America if they had. US imperialism has indirectly help start more wars than most Americans realize.