Nothing could have better pleased Sviatoslaf than to assist the Greek Empire, and when he captured the Bulgarian capital city on the Danube, and even talked of making it his own capital instead of Kief, it looked as if a great Slav Empire was forming with its center almost within sight of Constantinople.

The Greeks were dismayed.

With the Russians in the Balkan Peninsula, the center of their dominions upon the Danube--with the Scythian hordes in the South ready to do their bidding--and with scattered Slavonic tribes from Macedon to the Peloponnesos gravitating toward them, what might they not do?
No more serious danger had ever threatened the Empire of the East.

They rushed to rescue Bulgaria from the very enemy they had invited to overthrow it.

After a prolonged struggle, and in spite of the wild courage displayed by Sviatoslaf, he was driven back, and compelled to swear by Perun and Volos never again to invade Bulgaria.
If they broke their vows, might they become "as yellow as gold, and perish by their own arms." But this was for Sviatoslaf the last invasion of any land.