Although America and Belarus have yet to make an interaction in canon, Himaruya has mentioned in the strips Russia's Big and Little Sisters that Belarus established diplomatic relations with America, noting that she soon became clingy and America was wondering how he could get rid of her.
Christmas 2011 Event
After the arrival of roughly 100 people at the Axis Powers' party, Seychelles points out that the two most enthusiastic about Christmas, America and Finland have yet to arrived.
Belarus answers by growling that she's sure that he's exhausted from getting too enthuasiastic with the setup, refering solely to America.
Many fans portray this pairing as Belarus would be the type of girl who needs someone to tell her what to do, and America is the type of guy who needs someone to tell him what he's doing wrong and keep him in line.