The heavy use of slave labor brought about economic decline. Small farmers, who could not compete with free labor, went out of business. These former farmers were forced to make their way in the cities, increasing overcrowding. Slaves were often war captives from barbarian tribes used to independence, so slave insurrections, many that came close to immediately toppling the empire, were always a looming threat.
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The cause of the Roman empire expansion was successful military campaigns. The effect was that as the Roman empire grew, so did the number of enemies.
The reign of Caligula is mostly noted for his cruelty, sadism and sexual perversion. He worked very hard at removing any legal restraints that prevented the emperor from exercising unconstrained power. He spent lavishly on luxurious housing for himself. However, he did annex the Kingdom of Mauretania, and constructed two new aqueducts during his reign. He was assassinated in 41 AD by his Praetorian Guard.
Famine and disease spread throughout the empire
The invasions by the Germanic peoples precipitated the fall of the western part of the Roman Empire. They eventually took over all of its lands. The eastern part of the Roman Empire was not affected by these invasions and continued to exist for nearly 1,000 years.
This is an assumption, not a fact. Contrarily, high inflation towards the end of the empire is arguably the sign, and not the cause, of the weakening empire. See the reasons for the inflation, at the link below.