The basic idea is to rearrange history so that Thebes and not Athens becomes the dominant cultural power by the fourth century BCE.
One of the primary reasons why Athens becomes the dominant power in this period is their leading rôle in the second Persian war. So from this the idea arises to tweak the Persian War so that Thebes plays a leading rôle instead.
This runs a huge risk, of course, of being either very tricky, or very tacky. Besides of course "what if the Nazis won WWII?", one of the favourite questions of anyone considering alternate histories is "what if the Greeks won Thermopylæ"? The answer, of course, is that the Greeks couldn't have "won" Thermopylæ, and, more to the point, tweaking Thermopylæ alone would mean nothing to the (rather more important) naval battles.
With all that in mind, I set out to tweak Thermopylæ anyway. One of the main problems, of course, is that Bœotia as a whole—with the exceptions of Platæa and some other states, had allied itself with the Persians. But there turns out to be a very clever fix for this. Arguably one reason for this was a general hostility between Thebes and Athens, which itself was due to the aliiance of the Bœotian city of Platæa with Athens. This alliance can be traced back—on shaky ground, to be sure—to a counsel by the Lacedæmonians, who just happened to be passing through the area. And the Lacedæmonians were passing through on their way back from Athens, having been indirectly requested to come there by the Alcmæonid Cleisthenes in order to overthrow the Pisistratid Hippias.
So, to put it all together, this is the basic causal chain of events:
- Cleisthenes bribes the Delphic Oracle to tell the Lacedæmonians to overthrow Hippias.
- The Lacedæmonians go to Athens and overthrow Hippias. On the way back, they suggest to Platæa to seek an alliance with Athens.
- Platæa seeks an alliance with Athens, in order to escape Theban hegemony.
- Thebes and Athens end up on hostile terms.
- Thebes (and Bœotia in general) sides with the Persians during the Second Persian War.
- Athens, on the other hand, plays a major role in the defeat of the Persians.
- Athens enjoys widespread Greek hegemony.
The basic idea, then, is to alter this chain of events at the beginning, by, say, assassinating Hippias beforehand and saving Cleisthenes the trouble. This idea, of course, doesn't come out of nowhere, because that's exactly what Harmodius and Aristogeiton tried to do—in fact they tried to assassinate Hippias and his brother Hipparchus, and only succeeded in assassinating the latter. So all that's necessary is to make their assassination attempt a little more successful
, and this disrupts the entire causal chain:
- Cleisthenes takes leadership of Athens, thanks to the doubly celebrated Tyrannicides.
- The Spartans stay put (well, relatively speaking).
- Platæa falls to Theban hegemony.
- Thebes and Athens have no reason to end up on hostile terms.
- Thebes sides with Greece during the Second Persian War.
- In particular, with Theban support at Thermopylæ and Artemisium, the Greeks are able to hold out much longer in Bœotia, and the Persians probably never even reach Athens.
- Thebes enjoys widespread Greek hegemony.
[It's a stretch. I won't deny it's a stretch. Greece has to get really lucky to pull it off, and Greece was already
really lucky. But it feels clever enough that I'm sticking with it.]