Answer:
The main negative effect of imperialism includes, but is not limited too:
1) Usually the invading country does not allow for build-up of military or even police forces. When the invading country leaves through emancipation of the host country, the host country usually has little to no national defense, as well as equipment, if any.
2) Economic breakdown, in which the invading country prioritizes cash-earning industries such as cash-crops, oil-drilling, etc., rather then focusing on building the framework of a successful economic standing. When the invading country leaves, many of those who specialize in these jobs will also leave, leaving empty shells of profit.
3) Infrastructural breakdown, in that many of the infrastructures are not suited for growing populations, as well as ordinary people to live in. For example, Imperialists may build a large plantation house coupled with small shacks for servants and workers to live in. However, following emancipation, such infrastructure would not be suited for housing multiple families.
4) Governmental breakdown, which does not occur all the time, but sometimes happen, especially when the invading country does not break apart natural enemies, and places them together. This would lead to large-scale warfare between groups that would result in instability in the area, further detrimenting the countr(ies).
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Simply just state these, and explain point by point, giving one paragraph for each point, and follow it up with an example.
Good luck.