The Background scenario has been deliberately kept vague because what little we do find out makes no sense. It requires every nation Australia has as Allies to fail in its treaty obligations. Not just the United States, But the Commonwealth of Nations Lead by The United Kingdom and even France have defensive obligations.
Perhaps the others aren't wiling to help because the United States is unwilling.But Their big reason for not helping is about a Vietnam style quagmire occurring. This is proved Moot: Australia's an ISLAND Simply cutting off the invaders by Sea and Air;with no supplies the enemy would be forced to surrender.
Well, yes and no. It's true that other countries would definitely come to our aid, as Australia exports a vast amount of mineral resources to the rest of the world. However, just because Australia is an island doesn't mean it's easy to liberate - the entire Pacific arena of WWII consisted of mostly islands but it was still a long and difficult war.
New Zealand comes to Australia's aid. And somebody must have given/sold us some combat jets in a big hurry, because we don't have any at the moment but we do in the books. (But the books may have been written when we did have combat jets - they were retired sometime around 2000.)
It was, the first book was published in 1993.
The Invader's logistical magic
I can't understand how an invasion of Australia would even be conducted from the point of view of organizing the logistical support, resource distribution, being able to maintain the flow of materiel or personnel to the warfront and being able to keep it secret from every other nation. As the troper above noted; Australia's an ISLAND so the naval fleets that are supplying the ground forces are pretty damn isolated, unless its Indonesia, regardless of which, its such a massive hole that it detracts from the whole plot. Heck it ties into the above because it's unlikely the US Navy's Pacific Fleet is just going to ignore the massing of ships off the cost of Australia!
Australia's an island, but it also has a large amount of its own resources - which is exactly why it's such a tempting target in the first place. Any invading army would immediately seize farming areas - which they did in the books/movie - and use them to feed themselves. Not to mention that nations have been waging sea invasions since the dawn of time, many of them successfully.
The logistics are made even more difficult by the fact that the geography of Wirawee indicates it is thousands of km south of any potential enemy base - the filming was done in New South Wales.
Okay Ellie, I can understand you tearing Chris a new one for falling asleep on watch - even though he's a stoner, he's seen a family murdered and he risked everyone's lives. But then later on, you and Fi decide to sit in the petrol tanker to gossip about boys so long you don't notice the radio is off and a bunch of soldiers are coming up right behind you. And then you both forget the lighter in a ridiculous 'I thought you had it!' exchange. All of this directly leads to Robyn breaking her vow of pacifism and getting Corrie shot. So who exactly risked everyone's lives because she didn't goddamn check the big side-mirrors and keep her mind on the mission? Oh, you. And it was Chris who saved your ass. Way to fucking go, genius.
Part of the appeal of the books is that the heroes aren't supermen who get everything right. Sometimes they stuff up. Some of those stuff-ups cost lives. But yes, this was pretty egregious.
While Ellie was right to blame Chris, part of the point was that she went too far. Everyone makes mistakes, as Ellie and Fi soon did, and threatening to shoot someone is not the way to handle it. Also, note that in the books none of this happens. The radio breaks and Corrie and Robyn are completely uninvolved in the destruction of the bridge.
Surely, I cannot be the only person who views this series as pretty much a rip-off of Red Dawn (1984)? A country gets invaded, and who ends up fighting the invaders in a guerrilla war? Why, the local high schoolers of course! Adding in the fact that at least the premise of Red Dawn is a little believable (The Soviets were a fairly reckonable force after all) since with America getting invaded, the Soviets could have trashed all the US's allies, or taken them as allies. Whereas, the Tomorrow series has a foreign power (Who we never find out exactly who it is) invade Australia, without invading anyone else. Ok, for one thing, what, (if any) strategic value does Australia possess? I'm Australian, and even I realize that invading here is pointless. All the major cities are on the coastlines, the coastlines are fucking huge, with a lot of uninhabited areas. The population is pretty paltry, for a landmass of this size. We have no nuclear arsenal, no major supply stockpiles. We have nothing of any real value (Like fuel or whatnot) We have got a few large diamond/gold/copper mines, but if you're gonna invade a country for that, you're pretty damn stupid. The whole series is patently stupid, and like I say, a rip-off of Red Dawn. And yes, I did have to read the first two books for school, so perhaps that could be part of my reason for hating it.
The reason behind the invasion of Australia is, ostensibly, the sheer amount of unused space. Compared to other countries, Australia has a lot of unused space, which could be used for something, only because Australia only has a relatively small population, no one is using it. The reason put forward in the books is that the invaders, whomever they may be, come from a country with a much more dense population, and intend to use all the excess space in Australia for farmland and, well, living in.
That looks good on paper, but it's wrong. Australia has a large land area, but most of it is desert. Even arable areas are prone to drought, and just trapesing in saying "Look at all this land going to waste, I can farm it way better than you" is just asking for trouble, as the First Fleet learned. This fact is (to the best of my knowledge) mentioned exactly once in the series, when Ellie sees that the colonists are farming much more intensively than the Australians ever did (multiple families in a farm formerly occupied by one) and thinks they'll see trouble later on when the rains aren't so favourable. Then she takes it back, thinking maybe they know what they're doing.
Assuming of course that invasions are mounted for logical reasons. There wasn't much logic to Hitler's invasion of the Soviet Union, except for ideological reasons. Or the Falklands War. Perhaps the idea of invading a large country when you have an overcrowded and restive populace made more sense to the politicians who ordered it than the soldiers and logisticians who had to carry it out.
Eh, I hate to say it but the invasion of the Soviet Union and the Falkand Islands were made for eminently sensible reasons, albiet very, *very* evil. The Argentine dictatorship wanted to stay in power and sought to do that by getting the population to rally around the flag, and decided that if they were going to do that they might as well try to recreate the Viceroyalty of Rio de la Plata. Hitler invaded the Soviet Union to carve out a Germanic empire, to further isolate the West, and above all to destroy a crushing ideological and racial threat before that threat had time enough to modernize and attack *him.*
That's my theory as well. The leader in charge of Whateverplace, desperate to hold onto power, decided that a war was the best way to provoke Patriotic Fervor and quell the opposition, and that Australia, possessing a lot of natural resources (see the next paragraph) as well as being a very wealthy nation (and therefore "imperialist"), was a tempting target. In the movie, the radio broadcast said that Whateverplace was invading Australia for its resources in general, which sounds better than the book's explanation that they invaded for arable land (Australia's facing problems with overpopulation and water scarcity already). It's similar to a WMG I read for Battle: Los Angeles — Whateverplace has been pushed to the brink by resource scarcity and unrest, and attacked Australia out of desperation.
And about Australia's natural resources, and what the OP said about them: the country has a lot more than just gems and precious metals. Australia is home to some very large supplies of iron, coal, nickel, aluminum, uranium, rare earths, zinc and, yes, copper (it's a lot more important than you realize) — basically, the foundations of industrial civilization. If I were, say, an alien general leading an invasion of Earth, Australia would be near the top of my target list for that reason alone.
As for why Australia, keep in mind that it's not just a deathtrap. It's a very valuable deathtrap. For one, it's a major Western Ally, a military and industrial powerhouse in the South Pacific, and home to very valuable shipping lanes, resources, and all that land even if a lot of that is marginal. Knocking it out would radically destabilize the West overall and pretty much win the Southwest Pacific for the aggressor. Also as for the population, it's not that fargone as you think. Both of the probable aggressors- China and Indonesia- have population crowding issues, and while the former has a rather empty (by comparison) West, Indonesia doesn't. A big part of the reason why the Indonesian nationalists sought to and eventually did gain control of the entire Dutch East Indies was basically to alleviate overcrowding and resource shortages on the home island of Java. If they somehow exhaust their own resources again, they could do a lot, *lot* worse than going after Australia particularly if they are that advanced as the book's (incredible) logistics indicate.
Major spoilers for books 4 and 5: If aviation fuel trucks react so violently to small arms fire, why did the Kiwi's try to infiltrate the base, rather than just snipe the trucks from 3km away? (This isn't quite annoying enough to put on the main page under 'fridge logic'.)
Spoiler for Book 4: When they came up with the brilliant idea to send these kids back to Australia to act as pathfinders for the New Zealand commandoes, why didn't they equip them with, oh, say, uniforms and false identification? Much as I hate to admit it, the enemy had every right to shoot them out of hand on capture instead of hauling them off to Stratton Prison in Book 3; they were waging war out of uniform and were not legal combatants under any version of the Geneva Conventions. And they were still under sentence of imprisonment or death (Ellie and Homer); giving them a chance to plausibly claim to be legitimate ANZAC soldiers and to give false names in the event of capture would seem like basic good sense.
I honestly don't know, but I'd guess at this point the Enemy's behavior led them to question if it'd be worth it. Secondly, it might lead to bad press if it ever got out about the lengths they were going to to get manpower into arms. Thirdly, it's also likely that the Enemy was operating under some version of the Commando Order, meaning they're fucked no matter what they do if captured.