By the time they are powerful enough to take Australia, what makes you think America would be left untouched? They’d have all the material resources they’d need, to match their industrial capacity to build the fighting machine necessary to take over.
Add to that a colluding, runaway national government, and what is it that you can do? Please don’t tell me about civilian version, crippled, 3-shot burst “assault” rifles. The Afghans awash in automatic weaponry, RPGs and shoulder launched missiles have no chance against pilotless drones - the same ones the US government is building up local stocks of, for domestic use.
If "they" ever take America, likely there won't be anything left to justify occupation.
A rifle behind every blade of grass, unlike Australia (with little grass and few rifles) leaves only the use of weapons of mass destruction as a means to defeat US. The outcome of which the Bible describes in the destruction of Mystery Babylon, I fear.
Whether this be major cities or the entire nation, remains a mystery.
As far as taking America by conventional means, forget it. We have the ability to move a minimum of 10 million well equipped and well armed personnel to any part of the country within a 48 hour notice.
I do sincerely regret, under the current administration, we probably won't be able to assist you, as we were able to do in the past under more capable administrations.
G' day mate...