The Battle of Belleau Wood. June 10 to the conclusion.

On June 10 the Marines resumed the assault in the Belleau Wood, but without success.  The Germans resorted to using massive amounts of mustard gas to blunt the Marine attack as well as devastating fire.  That didn't keep the US from resuming the attack the next day very early in the morning, however, to greater success. That June 11 attack also incorporated U.S. Army units of the 2nd Division.

Over the next few days the Marines attacked again and again, until finally on June 26 the Wood was reported as being in U.S. hands.

US casualties had been severe, with over 1800 killed and 7900 wounded.  German losses remain unknown but 1600 Germans surrendered over the course of the battle.

The battle was a significant one for a variety of reasons.  For one thing, it was the first really large scale American operation of the war in France.  It was not the first time Americans had been in action by any means, but this action involved two US divisions and a French division under American command, although most of the fighting fell to the Marines in the 2nd Division.  Five German divisions were engaged at various points during the battle.  And it was the successful test of the American theory of combat which varied enormously from that which was being applied in the British and French ranks.  American troops had successfully turned and reacted to a German assault and then they had declined to retreat and declined to entrench.  This reflected the views of American leadership which was critical of the way that the Western Allies had fought the war to date.  Instead the Americans fought a modified large scale version of war the way they had practiced it since prior to the American Civil War.

Moreover, the Americans, while green, proved to be a tenacious enemy to the Germans which the Germans themselves noted.  By this stage in the war few units on either side were willing to engage in risks to the degree to which the Americans were, to the German's surprise.  American rifle file, moreover, proved to be highly accurate, reflecting American marksmanship in general and also the fact that most of the troops engaged in this battle on the American side were Marines.  Indeed, while the American forces were green in general, the unusual use of Marines as regular infantry changed the way that they'd be ever after used in the American military and pitted the Germans against a force whose NCO corps was incredibly gruff and in fact experienced.

Whether the US would have been able to continue fighting this way indefinitely has often been questioned.  Pershing's theories about fighting in France were vindicated in this battle, but that does not mean they were proven. They certainly took the Germans very much off guard.  But as troops became more experienced it is questionable if they would have been willing to continue to fight so recklessly.  On the other hand, American troops were now flooding into France with fresh divisions and they'd continue to be fresh for quite some time.

At any rate, Belleau Wood was a seminal American victory.  An American division had stopped the advance of more numerous Germans, and then a second American division had driven them back.  While the Germans continued on with their Spring offensive, the offensive advanced no further here.  The German Spring offensive was, in fact, reaching an end just as American troops were arriving in large numbers and engaging in action very effectively.