Australia only came into existence as a nation in 1901 and didn't start exercising an independent foreign policy until 1941 (as a dominion of the British Empire, we had independence on domestic issues, but foreign relations were still handled from London - we acquired the right to exercise an independent foreign policy in 1925, but did not take advantage of that until 1941 and our first action on doing so was to formally ally with the United States.
The first time Australian and US troops fought together, however, was on the 4th of July 1918, when four American companies were incorporated into the 4th Australian Division for the Battle of Hamel - the date was not a coincidence, the Australian commander, Lieutenant-General John Monash deliberately set the date because he thought it would help inspire the American troops he'd been given (for most of them, it was their first time in combat). The Battle was a complete victory for the allies. Corporal Thomas Pope of the 131st Regiment of the US Army was awarded the British Distinguished Conduct Medal on the recommendation of General Monash, and subsequently received the Medal of Honor for his actions during the battle.
Ah, thanks. I just woke up a little while ago and my brain isn’t quite in gear yet.