Three things.

Firstly, you can't assume the ability to win is measured in sheer number of men or tanks alone. See this video for a hint at what I'm getting at:

I don't know if those 'united islamic state' forces you speak of are going to happen anytime soon if all they appear good at doing is fighting each other and picking off a few of our civilians at a time in small scale terror attacks. The forces that control US CIA and war machine seem better at fomenting conflict in the region than islamic countries are at presenting a united front against us.

Second, if it came to civilians defending the country, you don't have a united country in the same way it was since mass immigration / multiculturalism has been imposed. You've got multiple races and religions who may exhibit the veneer of getting along in peacetime, but will this continue when the going gets tough, resources are scarce and it's the law of the jungle? Think Yugoslavia (and they're basically the same blood). Would first / second gen Asians here fight for their investment property or their own homeland and people? How many of them would be secretly working for the other side? Even among whites, the cultural issues that divide, arguing incessantly over things like gay marriage, transgender bathrooms and the right to wear a burqa - who's going to risk their life and pick up a rifle to fight for that?

Third, this will likely seem obvious, but states versus states is getting less relevant, outside mainly Russia and East Asia. For Arabs, as explained in the video, religion and clan come first. As Western countries become, by design, proposition nations no longer defined by ethnic heritage, (hence a recent Muslim arrival from Africa with a passport has just as much claim to be British or Australian as any white Briton) the concept of Australian, American or British will cease to mean anything, and perhaps a Pan European nationalism and sense of brotherhood based on race will emerge, leading even to 'you attack one of us you attack all of us'. There are other ways to enslave, dumbing a population down through education system and media owned by a hostile elite, cyber attacks to infrastructure, terrorism, psychological warfare, etc. And why invade a country when you can keep buying bits of it till there's practically none left? No need for bloodshed, and it happens over generations so you don't notice.