I'm not sure this is necessarily a question for historians, since the Industrial Military Complex is very much alive nowadays.

To put it simply, it is an alliance between business (here industrial), and military spheres, with government intervention being an important part of the issue.

It is not born out of a shadowy conspiration for war and profits, but does feel that way, because of how it developped and how it functions (and isn't that far from it...)

Basically, a system arises, in which both of these sphere's power increases and their interests get mixed, the World Wars being a great example of that. These situations usually create a need for a much stronger military which in turn demands an industry in order to fuel that capacity. Said industry, the military industry, has to specialize, therefore creating a co-dependency between the military, which needs power and the tools for that power, and the military industry. Indeed, many of these tools will be supplied by the industry, which itself needs profits it can only really get from the military's demand for its products. Both systems are not separated at all, creating all sort of mixed interests, with former soldiers using their networks in the military to set up successful businesses, politicians having decisional power in the military domain along with vested interest in the industry...

So what happens is that you get a system in which both players that got powerful each reinforce each other, the issue being that the industry reinforcing the military means an artificially increased need for military interventions, and the military reinforcing the military industry means an artificial growth for that industry (artificial growth never really being good in economic terms)