The Third Century Crisis
In the 200's AD (the third century), the Roman Empire had a very hard time. Historians disagree about the reasons, but certainly one reason was that the Romans were constantly being attacked by the Sassanids in the East and at the same time by the Germans in the North. It was very expensive to fight two wars at the same time, so that taxes had to be very high, and people were unhappy at having to pay such high taxes.
Numerian
Also, the Roman Emperor could not lead both armies at the same time.
So he always had to choose some general to lead the other half of the
army. Either the Emperor fought the Sassanids, and the army fighting
the Germans had only a general to lead them, or the other way around.
But whichever army only had a general to lead them would get jealous.
Often they tried to make their own general the Emperor instead. Sometimes
they won, and sometimes they lost, but this meant that the two halves
of the Roman army spent a lot of time fighting each other instead of
the Sassanids and the Germans.
Trebonianus Gallus
And, because the ordinary people were so unhappy about
their high taxes, it was pretty easy to get them to revolt too. So there
were a lot of smaller rebellions during the 200's AD,
and the armies had to fight them as well.
Zenobia
Because emperors kept being killed in battle or by the rebellious soldiers, there were many emperors during this century. Hardly any of them lasted even five years. So we will not bother to list them all here.
For more on the third century crisis, check out these books from Amazon.com or from your library:
Classical Rome, by John Clare (1993). For kids, the whole political history from beginning to end.
The Roman Empire, by Don Nardo (1994). For middle schoolers and high schoolers - from Augustus to the fall of Rome.
The Romans: From Village to Empire, by Mary Boatwright, Daniel Gargola, and Richard Talbert (2004). Okay, it's a little dry, but it is up to date and has all the facts you could want.
The Roman government's response to crisis, A.D. 235-337, by Ramsay MacMullen (1976). MacMullen is a very well-known and well-loved historian, though this isn't his best-known work.
Palmyra and Its Empire : Zenobia's Revolt against Rome, by Richard Stoneman (reprinted 1995).
The Roman Empire from Severus to Constantine, by Pat Southern (2nd edition 2001).

