In May of 1944 American forces under the command of General MacArthur invaded the island of Biak off the northern coast of New Guinea. Their objective was three airfields the Japanese had built on the island. The landing went well and soon 12,000 American troops were ashore. But when they began their advance on the airfields they found themselves stopped cold by very determined Japanese resistance under the command of an exceptionally able officer, Colonel Kuzume. It took the Americans about a month to finally capture the airfields. During that time, the Americans were very short on air support. All the US carriers were off preparing for the invasion of the Marianas and the closest land based air power was several hundred miles away.
Meanwhile, the Japanese high command had become very alarmed by the invasion of Biak and had devised Operation KON to relieve the island. The forces devoted to this included three battleships (Including Yamato and Musashi), a half dozen cruisers and numerous destroyers and transports. They had also assembled about 200 land-based aircraft in the region. The only Allied naval forces in the vicinity were three or four American and Australian cruisers and about a dozen destroyers.
Fortunately for the Americans, the Marianas operation began before Operation KON could really get underway and the Japanese quickly diverted all the KON forces northward to meet their fate in the Battle of the Philippine Sea.
But what if KON had been launched sooner, or the Marianas operation was delayed? The Japanese forces could have swept aside the Allied ships and heavily bombarded the tiny American beachhead and all the supplies stacked there. Caught between Colonel Kuzume's forces and the reinforcements coming by sea it is not impossible that the American troops would have been annihilated.
A disaster like this was not going to substantially change the course of the war, but what about the political ramifications and the affect on MacArthur's reputation? The general would probably have tried to put the blame on the Navy for not giving him more support and that might have found some sympathy among the American public. But would MacArthur still have retained the sort of clout he had with the military and Roosevelt? Would he still get his way and be allowed to invade the Philippines? Or would his command become a dead end backwater while Nimitz's drive through the Central Pacific became the main (and perhaps only) road to Tokyo?