The military industrial complex is the tightly linked relationships between military and industry. They are a relatively small group with close personal relationships. It is very common for the military leadership to transition from the military to the industries with which they worked.
The reason it is dangerous is that prior to when senior officers (especially) resign or retire they have a ready made job with the company they were formerly in charge of approving. That means that they favor no bid contracts, have favorite contractors and exert an undue influence on what goods or services are purchased. When they go to the industry, chances are that the military they work with were their former subordinates.
Another danger is that new fancier weapons are developed speculating on the next war or conflict. If you buy those weapons, you are itching to use them. It is understandable because it’s like buying a new car. Would you want to just park it in driveway or would you rather take it out on the highway to see what it can do?
This leaves the military leadership with a desire to go to war and an industry that fully supports them. When you have incompetent civilian leadership, like our last president, he leaves big decisions like wars to the generals. Generals that have a stake and future in bigger and better wars and weapons.
Additionally, these industries are now international. It is very likely that the arms suppliers are supplying both sides in the conflict. The bigger the war, the better the profits.