Well, I would call Bagration pretty devastating, a death blow.
As smashing as a victory as Bagration was, it still falls short of constituting a “Decisive Battle”. The Soviets only conceived it as but one part of a much larger planned sequence of campaign plans. What it
wasn’t was something that won the war in a single stroke right there-and-then, which is the definition of “Decisive Battle”. It still took almost another year after Bagration and millions of German and Soviet (and other Eastern European citizens) lives until Germany finally capitulated.
The WW2 battle that really comes closest to fulfilling the strict definition of “Decisive Battle” is Germany’s 1940 campaign in France. In the summer of 1940 the Allies had the advantage over Germany by all conventional means. They had more people, a larger combined economy and individually healthier economies, access to more international markets, and at worse numerical parity in soldiers and weapons on land, with gross numerical advantage in naval power. They were producing weapons in increasingly greater quantities than the Germans, and had the situation continued without change for another year the Germans would have been at a unbeatable disadvantage.
Instead in May of 1940 the Germans attacked. In no more than three days of battle, from 13 to 15 May, the bulk of the German panzers broke through the French line on the Meuse river between Dinant and Sedan, and within a week had encircled the best of the Allied armies, destroying them or putting them to flight. Although it would take several more weeks for the Germans to finish crushing the French Third Republic, the war had been decided.
Except it hadn't been. Only the battle was decided. Britain fled the continent, but remained in stubborn opposition off the coast of Europe. Indeed, had French PM Reynaud gotten his way the French government would not have surrendered but would have fled into exile in Algeria and continued the war from the colonies. The Germans had executed one of the most brilliant battleplans ever devised, applying all the usual maxims about concentration of force and minimization of friction like masters, but it had still failed to win them the war. Worse, the Germans had been so focused on the immediate battle with France they had made no plans for the prosecution of the war after winning it. This left them unable to effectively follow up against the UK, and contributed to the German High Command's willingness to blunder into Russia (in addition to Hitler's desire for eastern expansion, many German generals believed defeating Russia would force Britain to negotiate peace), and their ultimate defeat.
The battle of France was about as decisive as a battle can be, taking a side at a disadvantage and then reversing that situation in one transcendent blow. And yet Germany was still unable to use it to win the war. That suggests that the concept of the decisive battle is inadequate, and one needs more than just winning battles to win a war, particularly if one is the weaker party. Weaker nations have certainly defeated more powerful ones several times in military history, but (certainly in modern times) they rarely do it through winning a decisive battle. More often victory comes through the manipulation of multiple elements of the situation, in which battle often turns out to be one of the less important ones.