Well, if America had joined the Great War, everyone could have seen that it was a military great power (or at least had the potential to quickly become one, if it wanted). For decades, US military was comparable in size to Bulgarian or Belgian military, and quality was lacking too, so America looked like a paper tiger, but Americans had never needed the military power as they mostly weren't interested in foreign adventures (except in Caribbean where their existing military was enough), and were protected by oceans.
Also, if America had joined the war, Russia, Britain and France might still have lost to Germany in Europe, but wouldn't have been so utterly crushed (it would have been more a "defeat with honor") nor would have collapsed economically.
As a result, Japan would have been unable to gobble South-East Asia from collapsing French and British Empires (as well as the Dutch who couldn't resist without Entente support), or to bring China to heel (as Russia, Britain and France would have, of course, interfered). Japan would have been much less successful. They would have conquered Korea and Manchuria, for sure, but not much else.
And ironically, that might have saved Japan ultimately. Alas, stupid Emperor Yoshiro and the IJN (with full-on victory disease) thought "Hey, we have defeated Russia in 1905, and then after the Great War, we acquired all South-East Asia from Europeans... How difficult can be an invasion of Filipinos ? Americans are wimps anyway". And then, they learnt the hard way that Americans are not wimps.