Questions about Hegemony and Decline

Hello,

In a sense I have a very simple, and perhaps simplistic, question that I
find difficult to address with the aid of existing theories and analytical
frameworks. I would like to believe that there is much that I have missed,
that this question has already been addressed, and that someone can help to
shed light on this.

The question is this: how is it that small shattered countries can be the
undoing of "great powers" such as the USSR in the case of Afghanistan, and
possibly the USA should it invade Iraq? How could a small, divided, and
battered nation such as (North) Vietnam have been so successful? What I mean
is, "hegemony in decline" or not, all of these countries faced overwhelming
odds and, on the face of it, should have had no chance of succeeding let
alone inflicting lethal wounds on their aggressors.

Unfortunately, I stopped reading theories of imperialism years ago. If I had
continued, I would have liked to see some revision of such theories in a way
that debunks the alleged powerlessness of the small, the poor, and the
peripheral.

Can hegemony be measured by the size of one's nuclear arsenal, for example,
or can it be measured by the number of box cutters one owns? Maybe this is a
silly question, but it's the kind that plagues me.

Cheers,

Maximilian C. Forte