No greater cause of war has ever existed than a disproportion between countries or tribes of such a character that one was rich and weak, while the other was strong and poor.

Nations are much like individuals--and not very good individuals.

Highwaymen who are poor and strong organize and drill for the purpose of attacking people who are rich and weak; and while one would shrink from declaring that nations which are poor and strong do the same, it may nevertheless be stated that they have often been accused of doing so, and that some wars are explainable on that ground and on none other.
The wars of Caesar in Gaul and Britain do not seem to fall in this category, and yet they really do; for Rome was poor in Julius Caesar's day; and while Gaul and Britain were not rich in goods, they were rich in land, and Rome craved land.
Of course, there have been wars which were not due to deliberate attacks by poor and strong countries on rich and weak countries; wars like our wars of the Revolution, and with Mexico, our War of the Rebellion, and our Spanish War, and many others in which various nations have engaged.

The causes of many wars have been so numerous and so complex that the true cause is hard to state; but it may be stated in general that wars in which countries that were both rich and strong, as Great Britain and France are now, have deliberately initiated an aggressive war are few and far apart.
The reason seems to be that countries which are rich tend to become not militaristic and aggressive, but effeminate and pacific.

The access of luxury, the refinements of living that the useful and the delightful arts produce, and the influence of women, tend to wean men from the hardships of military life, and to engender a distaste for the confusion, bloodshed, and "horrors" of war.