The Australian-American War was a fictional war fought between the United States of America and Australia, mentioned in Grand Theft Auto III, Grand Theft Auto: Vice City, Grand Theft Auto: San Andreas and Grand Theft Auto IV. The war itself is implied to have occurred in both the 3D Universe and the HD Universe.
The war is first mentioned on Lazlow's radio show Chatterbox FM in GTA III when a veteran of the war, Col. James P., calls in to talk about how no one cares about the Australian-American War. Lazlow then goes on to mention that he's never heard of it. The vet talks about how the Australians equipped boomerangs and kangaroos with explosives.
Little in detail is known about the war. It is unknown who won the war, the Australians or Americans, or even if the war was actually fought, because of the possibility that it was made up in the caller's head. It is also unknown when the war took place, if it took place. If it did indeed take place, it's improbable that the war happened before the late 1950s, because the man calling called himself "Colonel James P., United States Marine Corps. Second Batallion", without adding "retired" to the end, and it's unlikely that an active member of the military in 2001 would've fought in the early 1950s or before. The caller also refers to the Australian-American war being "the biggest war since the big one", implying it occurred sometime after World War II. Also, the caller makes a reference to the fact that when he got home, everyone was watching TV instead of giving him a "hero's welcome". Television began gaining popularity as a norm in the middle-class American household in the late 1950s, and early 1960s.
GTA Vice City
In GTA Vice City, the war is implied to have occurred as far back before 1986, as it was mentioned in a radio commercial for Ammu-Nation. The commercials talks about an anti-aircraft gun "used when we whooped the Australians' ass!". This implies that the United States won the war. Kent Paul's 80s Nostalgia Zone website states that hostilities between both countries began in 1980.
GTA San Andreas
The war is loosely referenced early in GTA San Andreas after the San Andreas Earthquake rocked the state. In a WCTR news brief, Richard Burns said that the unnamed Governor of San Andreas was threatening to bomb Australia because of the earthquake, "...despite scientific evidence that it wasn't their fault."
The radio commercial heard in GTA Vice City can also occasionally be heard inside Ammu-Nation stores in San Andreas.
In GTA IV, the war is mentioned again in radio, confirming that the conflict occurred in both GTA III's and GTA IV's continuities. On WKTT, a conservative radio station, a caller to The Richard Bastion Show says, "This country's going to hell! Ever since we let those damn Australians back into this country, everything has gone to shit! What the hell did we go to war with them for in the first place!?" which could be another reference to unpopular wars like the Vietnam War. The host, Richard Bastion, replies "I'll tell you why, we did it for freedom, and for valuable munitions contracts."
The radio advertisements for Babiesovernight.com offer customers a "funny, but millitarily inept Australian baby", perhaps implying that the war didn't go too well for the Australian side. If one browses the in-game internet and accesses the actual Babiesovernight.com website, it is possible to see an offer for "an Australian baby that will love to play war but always loses", which may well indicate that Australia was actually defeated by the American side during the war.
In real life
In real life, the war never happened and is very unlikely to occur. Australia and the United States have a very close relationship having served alongside each other in every major conflict since World War I.
The closest to an Australian-American war was the Battle of Brisbane; in August 1942, Australian and American troops fought each other in a riot known as the Battle of Brisbane, in the Brisbane central business district. One person died and hundreds were injured. This doesn't entirely match up as it happened in 1942 and most recognize it as being the Australian soldiers who "won".