This is the question that history always wants answered.

I believe at TARAWA it was the AMPHIBIOUS TRACTOR, better known as the LVT.
Without the LVTs, the first three waves would not have made it over the reef, a beachhead would not have been established and large amounts of supplies, especially ammo, would not have gotten to the Marines on the beach.

The Japanese believed the reef would have stopped all landing boats, and it did. Without the LVTs the first three waves of Marines would have been devastated on the reef, follow-up waves in landing craft were. The momentum of the assault would have completely stopped, which it almost did except for the repeated runs into the beaches by the LVTs and thier crews.

Even though the momentum of the assault came to a crawl, the LVTs continued to bring in Marines and supplies to the beaches. They saved many Marines by taking the wounded out to the waiting landing boats just off the reef.

Out of 73 LVT-1's committed to the battle (2 LVT-1's were never unloaded) 56 of them were destroyed.

There were 50 LVT-2's committed- 34 were destroyed.

That makes 90 LVTs out of the 125 destroyed. Many of these LVTs operated after the initial landings before being destroyed by enemy gunfire, sank in the lagoon, mechanical failure or beach mines.

The fighting spirit of the individual marine won the battle, but it took the LVT to get them close enough to the fight.

That's my two cents, I am a little bias.

Happy Holidays!!
Al