Strange World Yellow connects to at least 900 realities that include some version of the United States of America. There are many Americas where the Confederacy won the Civil War, as well as Americas where the Civil War was fought in the late twentieth Century with tanks, supersonic jets, and ultimately ICBMs. There were Americas where Dewey was president, where the Nazis ruled east of the Mississippi, and even more than one America where global war had left ruins or melted rubble.
There are several thousand known Strange World doors connecting to these Americas. Americans were instrumental in re-populating Nexus City. English is one of the most common languages spoken in the Strange Worlds. Still, many Americans were not happy with the burgeoning inter-dimensional civilization connecting the three tunnels.
Presidents tried to keep the Strange Worlds secret. Congresses published disinformation about the threat represented by uncontrolled trade with other realities. Courts passed laws to restrict free migration through the tunnels. In many cases, it was a losing battle.
Eventually, the leaders in ten distinctly different realities met and formed the "United State of Americas." This coalition had several goals: carefully controlled trade, policing the Strange Worlds tunnels, and preventing the subversion of the American way of life. Within a decade, nearly fifty versions of America had joined the USAs.
Whenever possible, the USAs would intervene in newly discovered realities where America was under attack.
But there were so many realities and never enough time...