Byzantine leaders often paid foreign soldiers to fight in wars for them. What was the problem with this practice?

The main problem is that those soldiers were not loyal and would often turn their backs on the Empire. On the other side, Empire needed to spend a lot of money on mercenaries and thus weakened their economy.

Explanation:

When we take this fact into account we can see that actually the same thing happened in the period of the Late Roman Empire. Lack of soldiers introduces the need for mercenaries. This weakened Empire from the inside, and some of the mercenaries later turned their back on the Empire. For example, Turks Seljuks were important part of mercenary army and later made many problems for the Empire.

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