The battle really wasn't that big and it's fame, much greater then its actual importance, is due to a fluke.
The battle began when advancing German forces made contact with three battalions of the 5th Marine Regiment. There started a turkey shoot as the Springfield armed marines proved to be expert marksmen. Thus the German wave was broken.
Here's what we wrote:
While elements of the 3rd Division went into line on the southeast of the salient, the 2nd Division took up positions on the salient's southwest. There was no contact with the enemy, save for a periodic artillery barrage and long-range machinegun duels. The next day, the division relieved the French 164th Division west of Chateau Thierry. As French troops streamed toward the rear, Captain William Corbin of the 5th Marines was accosted by a fleeing French officer who advised the Americans to retreat. When Corbin relayed the message to Major Lloyd Williams, his CO, Williams replied in the most American way possible, 'Retreat, hell we just got here.' Williams’ simple statement of fact is today emblazoned across the 5th Marines’ shoulder patch.The follow-on offensive was less glorious. The Marines and army needed a few weeks to clear the wood. Over the course of the battle they learned John J. Pershing's notions of 'open warfare' were deadly on the WWI battlefield. The Germans scythed down the Marines like extras in an American WWII pic.
Throughout the next day, the Germans advanced through unspoiled fields against the marines. Here, Pershing's ideas on marksmanship were tested. One marine private bragged that, ‘A rain of good American lead from good American riflemen met them.’ A machine gunner described the action as only an American could, ‘Oh, it was too easy; just like a bunch of cattle coming to slaughter.’ Up and down the line the marines held the Germans back. Colonel Albertus Catlin described the scene, 'Then, under that deadly fire and the barrage of rifle and machine gun fire, the Boche stopped. It was too much for any man. They buried in or broke to the cover of the woods and you could follow them by the ripples of the green wheat as they raced for cover.'
 Aaron Eckhart says ‘Retreat Hell’ after a particularly daring exploit in Battle Los Angeles
 Catlin, 94-95
Anyway Belleau Wood became famous becuase the press, needing to add some color to the story, realized the more romantic sounding Marines were fighting and played up that angle. Also Belleau Wood was the second major action fought by the AEF after the struggle on the Marne.