Nothing like a good bit of USA bashing to get everyone’s blood pressure up, eh?
I think maybe our first question really ought to be how we're defining "greatest". Started the most wars? Started the wars that resulting in the most deaths? Won the most wars? Something else entirely? Because the answer might well depend on the question you’re asking.
There can be little doubt the WWI and WWII were the biggest wars the world has seen, and Germany has the dubious honour of being the aggressor in both of them, so I suppose that gives them a fairly solid claim to being the "greatest" aggressor.
Having said that, the US has been involved in a lot more wars, especially post WWII. Much smaller in scale, yes, but they've hardly been out of conflict since 1914. It'd take quite a bit of stats digging to actually say for certain whether all the wars the US has fought in adds up to more casualties than the two World Wars.
The quiet and reserved British Isles aren’t doing too badly for themselves in these stakes either. In a similar way to the US, the UK has fought a large number of wars and small scale conflicts almost continuously since the turn of the century. (Take a look a the Wikipedia timelines of
American Military History and
British Military History There’s a whole lot of wars and conflicts most people have never heard of.)
And lets not forget Africa (because far too many people do). Over the last century it’s been the continuous host to a very large number of wars, most of which we never hear of here in the west. Some of these last for decades, and I very much doubt anyone has tried to properly count the casualties. It’s probably safe to say millions have died fighting for causes we aren’t even aware of.
You might want to define "aggressor" too, since I think our governments would rather us not bill the Iraq wars as aggression on our part. We're defending any number of different things, liberating, fighting for freedom, for democracy, for whatever, it's a pre-emptive defensive move because he's got weapons, or wants them, is friends with terrorists, or might be. In my opinion, despite the old adage that "the best form of defence is attack", I don't think pre-emptive strikes count as anything but aggression. If I went around kicking the shit out of all the kids that hang out around the shops drinking, I very much doubt I'd be able to plead pre-emptive self defence on the ground that drunken teenagers sometimes get a little uppity. It'd be GBH and assault, and I'd spend some time in jail for it.
Maybe we’re asking the wrong question. Drawing our lines along international boundaries is silly. They’re just imaginary fences drawn by years of conflicts and built out of nationalistic propaganda that tries to tell us that “this side of the imaginary line is different”.
Humanity is without doubt, the most aggressive species on the planet. It doesn’t really matter where you’re born, what language you speak or what colour your skin is. We fight wars over whatever excuse comes to mind at the time, be that our various Gods and beliefs, our economic or political systems, over money, territory, oil, diamonds, over vague ideals like freedom and justice. We sometimes even fight them without any real attempt at justification beyond “because we can”. In the end it all comes down to the same thing. The struggle to be and prove ourselves, superior, and to crush any threat to that superiority. An evolutionary instinct gone haywire in our allegedly civilised world. Something we feel more strongly than any species on the planet. It’s why we are where we are, at the top of the food chain, the Alpha Species. It’s built deep into our collective psyche, intrinsically linked with the desire to survive. We’ve always fought. We always will.
The answer to the question “which country has been the greatest military aggressor” is simple: The one that had most opportunity.