The concept of 'decisive battles' is delusory to begin. Especially in recent times, outcomes are usually decided long before the battle, when the political decision to resort to war is taken. For example, although John Colvin thinks the decisive battle of the Pacific War was America's victory at Midway, the really decisive one was America's defeat at Pearl Harbor. Once Japan forced war, the outcome was never in doubt.
Nevertheless, ever since John Creasy in the mid-19th century, listing decisive battles has been a good way to sell books. Colvin's effort is at the bottom of this unimpressive list.
To begin with, the writing is atrocious, barely qualifying as English. Sentences and half-sentences wander around looking for the coherent paragraphs they might possibly belong to, like tourists lost in a subway looking for the right train.
Irrelevant factoids are dropped in. For example, Colvin tells us James Wolfe was paid 14 pounds per week during the siege of Quebec.
You might suppose that the factoids would at least be about decisive battles, but that would be wrong. The chapter on the Siege of Vienna has more about Polish King Jan III Sobieskis' harridan wife than it does about the siege of Vienna.
Is this the worst work of military history ever written? Probably not. It's a field that attracts cranks and incompetents. But Colvin's 'Decisive Battles' might make the bottom 10.