What lead up to the Battle?
The Japanese planed to extend their Pacific control and they would take Port Moresby. Control of Port Moresby would mean that Japanese would be safe from land-based air attacks and the Japanese Air Force would be free to attack other known areas surrounding the port. This would also cut sea links between Australia and the United States, stopping the movement of supplies and troops, leaving the Japanese with great victory in the Pacific allowing their forces to not be messed with giving them chances to attack regions nearby.
Why did it happen?
It happened because Allied forces were trying to stop Japan’s expansion and control of the Pacific Ocean area. The Japanese wanted to expand and with that they planned on attacking the U.S air forces. The Japanese launched an attack on the main American fleet at Midway, between Pearl Harbor and Japan, destroying it and forcing America out of the Pacific War.