Entirely apart from the fact that modern armies - at least, American ones - have protocols for reducing the effectiveness of tactical nuclear weapons (and since the circumstances of how to fight a war have changed dramatically since the Second World War - one doesn't focus on breakthrough, for instance - you can't use tactical nuclear weapons for the purpose of blowing a hole in the enemy's lines because there are no lines), just think about normal dispersal of force; it would take a unrealistically large number of nuclear weapons to be able to hit everything.
That, and there's no guarantee that the nukes would come out in any future global war anyway because of the wonders of second-strike. And the impracticalities of a military reliant on rockets and nuclear weapons were demonstrated quite graphically in the 1950s and 1960s anyway. Sure, if Poland and France had had nukes in 1939, they could've obliterated a few German spearheads and maybe turned the tide. But, you know, you need an actual army to take advantage of that, and they had one. Bombing a country into oblivion has literally never worked by itself, because people will always get around the bombs; you need boots on the ground to enforce your will.