When America was just a baby, England and France fought over who got to be his older brother. France attempted to win America over with food, something France was way better at making than England. However, America chose England after seeing him crying. England made sure to protect the baby, but after America effortlessly lifted a bison, England started to think America didn't need such protection.
In his childhood, America was fond of England, becoming sad when England had to leave to return to Europe. England made sure to give America toys and clothes. England slept with America whenever the latter was scared of ghosts, but this always resulted in England falling asleep quickly and America remaining frightened. One time when England left, he told America to grow strong, and the next time England returned, America was taller than him.
When America had grown up, he declared that he wanted to be independent, angering England. England overpowered America but was unable to shoot him, and instead collapsed into tears. Remembering how much he adored England as a child, America commented how England used to be great.
The two were members of Allied Forces during World War II, but did not get along. England and America often tried to humiliate the other or refused to help each other. One time, they were shipwrecked together and America said that being stuck with England was the worst fate imaginable.
England and America had a long-running tradition of scaring each other on Halloween, which England always won due to America's fear of ghosts until America got Japan's help.
USUK is a popular pairing in the Hetalia fandom. Its most notable rival ship is FrUK. On AO3, the pairing is the most popular ship for both characters and the Hetalia: Axis Powers fandom.
Many fans believe that England is a tsundere, acting cold towards America while secretly having feelings for him.