Midway.

This is coolbert:

“We had not been defeated by these superior Japanese forces. Midway to us at the time meant that here is where we start from, here is where we really jump off in a hard, bitter war against the Japanese.” - - Ray Spruance.

As extracted with commentary from the outstanding Internet web site isegoria.net:

"The Importance of the Battle of Midway"

"The victory at Midway aided allied strategy worldwide"

"Midway saves the decision by the Americans and British to focus their major effort against Germany, and the American and British military staffs are free to plan their invasion of North Africa."

THE DESTRUCTION OF THE KIDO BUTAI! The Japanese First Air Fleet. That assemblage of aircraft carriers and air crews that up unto Midway were absolutely unstoppable and ever-victorious. NOT ONLY THOSE JAPANESE SHIPS SUNK BUT THOSE HIGHLY EFFECTIVE AND VERY ABLE AIR CREWS IRREPLACEABLE!!

From the time of Pearl Harbor until Midway there was one Japan, very strong, in the aftermath of Midway there was a second much weaker Japan. That latter NEVER ABLE TO RECOVER FROM  MIDWAY.

Allied military planners, American and British both now able to concentrate 90 % of the war effort against Germany without further fear of Japanese advance.

WITH HINDSIGHT MIDWAY MORE IMPORTANT THAN REALIZED AND NOT WITH MERELY REGARD TO THE PACIFIC THEATER ALONE!

Yet more thanks to isegoria.net.

coolbert.