Rome's military was largely destroyed by Hannibal during the Second Punic War. In fact, Rome only held on by being better at resisting sieges, but Rome lost basically all their allies after their army was annihilated at the battle of Cannae. They later launched a battle in Carthage's backyard and it caused panic and disagreement, leading to Carthage's defeat. If the Redguards launch a similar surprise attack, the Thalmor are very likely to panic in a similar fashion.
I'm not exactly on expert on these things (I'm going off Latin class and History Channel, mostly) but weren't both Rome's resurgence and Carthage's panic somewhat endemic to their culture? Pyrrhic victories are named after the futility of destroying a Roman army, because Rome was such a fertile and populous land, and had a particular military culture that allowed the Romans to reform armies in a way the desert-bound Redguards probably lack. On the other side, the Carthaginians let too many leaders have a say in matters of war, and never gave proper strong support to the military.
And as far as the Redguards having the capacity, they likely have more capacity to now then they did after hostilities first ended. And from what I understand, they're not exactly terrible at naval warfare. A surprise attack is still a surprise attack, no matter your superiority.
Sure, they've recovered since they incentivized the Thalmor to end the occupation of Hammerfell, but there's a large gap between the military might required to, over the course of five years, get foreign troops to leave your land, and the military might required to launch a successful attack against your enemy.
The Redguards, AFAIK, are not at all known for their navy. The Dominion, on the other hand, is supposed to be the absolute best Tamrielic sea power.
And, quite simply, a surprise attack isn't always a surprise attack. If the Redguards go into Valenwood, they're going to have to fight the Valenwood. The native Bosmer are man-eating hunters by lifestyle. Most Redguards haven't ever even seen a forest. I doubt they'd be able to get the drop on the Dominion there.
At sea, again, the Dominion are famed for their naval power, and they know they have enemies lying in wait on all sides. Furthermore, they're a bunch of wizards with super wizard powers (including some forms of esp). I think it's likely they have sufficient sea patrols to spot any sort of Armada large enough to pose a serious threat to their islands. Though I'll admit, I could be completely wrong about this. I know next to nothing about how large navies spotted eachother before radar / sonar. I might be drastically underestimating the difficulty of screening against enemy attacks.
This is why it's unlikely the Redguards would attack unless the Dominion were already occupied.
Who said the Dominion would be fully occupied? I was suggesting they send out a few justicar squads to occupied Skyrim, or lend a hand (not an army) to the Imperial reconquest of Skyrim.
If all the signs are pointing to it, I'd say that it's about to happen.
Probably either the last DLC or something that happens between Skyrim and TES:VI