Hey Ariana!
The answer is: acquisition of the Oregon territory
With The Soviet Unions collapse, The US's main enemy that it had been opposed to for years was no longer occupying its time. Very quickly, the US turned its focus towards the many terrorist factions in the middle east, some of which it had facilitated. This lead to things like 9/11 and the Iraq war. Additionally, all the territory that had been previously been controlled by the Soviet Union near the middle East (Kazakhstan) were released and left to their own devices.
Historians have to overcome problems such as false reports of events and destroyed evidence
