It is sometime referred to as RusEng, taken from the first few letters of their nation names (Russia and England).
This pairing is believed to be a love/hate relation, often gory. Fans think the pairing was created when England "accidentally" summoned Russia, instead of the Devil.
It could also be seen as England still trying to get some sort of revenge on America, following the saying of "the enemy of my enemy is my friend." During the Cold War, England could've hooked up with Russia to try and form jealousy within his former colony. But didn't. And how did that end? America and him still argue all the time.
Other speculations include that England might be trying to fulfill his "maternal" insticts by caring for the more childish Russia
Sometimes, they are seen just bonding over the fact that they both were once huge empires that held great power in the world that are now not as powerful.