Ad: This forum contains affiliate links to products on Amazon and eBay. More information in Terms and rules
Actually, by early 1941, War Plan Orange (WPO3) had slowly been modified to where the push west across the Pacific was actually planned for about two years after the expected "surprise attack by Orange" (due to expected mobilization/modernization and expansion requirements) - and already included "island-hopping" to establish forward air bases and temporary supply/support naval facilities!
The "turn North to fight a decisive battle off Japan" part had been removed from the WPO series by the early 1930s, while Japan still planned on the US meeting them there.
In other words - the version of WPO-3 adopted by the US in April 1941 was virtually identical to what was actually done after Pearl Harbor.
So without the Pearl Harbor attack, but still with a Japanese invasion of the Philippines, there would have been little-to-no change from what the US actually did!
Only if Japan didn't attack the US anywhere would we see a significant change from history.
See this article: War Plan Orange
They walked right into it with their eyes closed at Midway only to find themselves on their heels when we opened their eyes. I think this is a fascinating question.Another question might be raised that after Pearl Harbor and more importantly, after the Battle of the Coral Sea, what if the IJN suddenly got a burst of insight and did NOT attempt to attack Midway? I believe Yammamoto was looking for an action where he could bring the USN carriers to battle to destroy them as they were the guarantors of the US/Australia/New Zealand pipeline, but perhaps would look for another avenue to achieve this goal.
If memory serves, even without a battle at Midway the US was going to invade Guadalcanal, so perhaps the IJN would have lost it's carriers in the battles around the Solomons. Although I think Nimitz was looking to take out the IJN's carrier fleet ( or most of it ) before the incursions in the South Pacific.