The point is not whether or not the initial instincts of the German (really, Prussian) general staff were correct, but what to do once the character of the war shifted against the Schlieffen Plan.
And? The two options were to keep on fighting France, and to stop fighting France. Neither option had been studied in advance, but both could have made tactical or political sense as per precedent.
The Klingon operation doesn't seem to have any of the characteristics of the Schlieffen plan, really: it's not a means of eliminating a player so that one can move on, it's not time-critical or resources-stretching, it isn't part of a broader scheduled campaign. There isn't even a von Moltke to mess it up, just some timely counterintelligence.
Worf said the Klingon Empire wasn't strong enough to fight both the Cardassians and the Federation, and since the Federation is certainly able to take the Klingons by itself, the fact that Cardassia was even worth mentioning indicates that the Union is in fact a significant power within Star Trek.
How so? Cardassia gets a mention because it's right there, the focus of the events. It doesn't need justification beyond that.
Cardassia is significant as the de facto guardian of the wormhole region (at least until the Federation really moves in), and as an old foe of the Empire. These are reasons enough for the attack, and apparently the only reasons. Cardassia being a significant military force is not a prerequisite.
Speculation aside, we have been shown in both TNG and DS9 that the Cardassian military apparatus was heavily involved in the administration and protection of colonies and annexed territory.
Only insofar as the colonies and territories were of interest to our heroes. Cardassians were involved at Bajor, but it doesn't follow that they would have been similarly involved anywhere else, and even Bajor was dropped like a soft and smelly tomato right before the wormhole was discovered. Cardassians were involved at the Demilitarized Zone, but it doesn't follow that they would not have had any forces left over for other projects - indeed, "Chain of Command" sort of proves the opposite.
As long as we hear of no other subject species besides Bajorans, we don't have to assume any need for colonial patrolling, British Empire style, either. All the other Cardassian Union worlds we saw or heard of were either inhabited by Cardassians, uninhabited save for Cardassian outposts, or left undescribed in terms of population and significance.
We might rather say the Klingons were the ones facing the problem of shifting their forces from full-time subjugation ("Mind's Eye" style) and proxy wars masquerading as civil unrest ("Redemption") to conquest. And the Klingon-Cardassian war need not have been part of the problem, but rather its solution: invade a weak enemy to regain the impetus of conquest and solidify the position of the leadership (and to cull the weak and the politically inconvenient).
Never conquered anything during the Talarian Wars?
Hmh? I'm sure the Union could have been conquering planets left and right during the Talarian, Erselrope and Punic wars. It's just that we never explicitly heard of any such thing (besides Bajor) in TNG or DS9 or VOY or the movies. Or ENT, for that matter.
Timo Saloniemi