Picture it: the year is 412AD, and the Roman Empire is on the brink of collapse. Rome is but two years sacked, Alaric and his Goths having rampaged up and down Italy at their liesure. A pretender to the throne lurks on the French/Italian border, backed by Britain and most of France itself, while barbarians of various persuasions are rampaging through France, ultimately to end up in Spain where they will claim the entire country for their own. The only strong West Roman army is tied up in Italy, frozen between the usurper and Alaric. Everybody is thrown into fits of despair.

And then! Constantius is nominated Generalissimo. By luck, the exigencies of war force the Goths into France due to the lack of supplies in Italy, and the new Roman general moves. He quickly defeats the usurper, sending his head back to Ravenna, and then the usurper’s follow up act, sending that head back too. Then he starves out the Goths in France, forcing them to kill their leader (Alaric is long gone by now, we’re talking Althauf, his brother), and make peace with the Romans. Canny and patient. THEN, he sends his new Goth allies into Spain, where they kick the other Barbarians upside the head and grab most of the land back.
Constantius! In quick succession he is made consul, patrician, consul again, supreme commander, married to the Emperor’s sister, and given control of everything. Everybody is full of vim and vigor, epic poems are composed, people toast each other, smiles all round. The Roman Empire is back! 
So far, so good. Epic, sweeping, dramatic. Just like a novel. This, however, is where reality asserts itself. A good author would now have Constantius turn about and prepare for the massive Hun threat. Ready the ranks. Instead, Constantius dies. At the peak of his glory, when Rome stands on the verge of consolidating all its gains, he dies a quiet, normal death in his sickbed. And the Empire is subsequently torn apart by internicine warfare, leaving it wide open for its outside enemies to take apart at their liesure. 
It would be like having Gandalf die of small pox just before he leads the Fellowship into Moria. Foul, would cry the readership, foul! However, perversely, it’s this kind of stuff that makes history thrilling to read. Because there are no narrative rules. Anything can happen, and sometimes? It actually does.