But if you insist...

The real prelude to McCarthyism (aka the second Red Scare) is at the very least the first Red Scare immediately following 1917's October Revolution in Russia. See for instance the Sacco and Vanzetti trials. We actually put troops on the ground in Russia to at least force the Russians back into WWI, and potentially to topple the communist government entirely (although the latter was more of a British ambition, and American troops were more or less strong-armed into helping). Arguably, the seeds for McCarthyism were planted as far back as the Paris Commune in 1871.

Its also worth understanding that the Red Scares were largely fanned by a widespread fear that the labor classes might rise up and seize the means of production, a fear fully justified by the Coal Wars starting in the 1890s. That the USSR and the CCP eventually supported labor organizations with a revolutionary bent is no real surprise, but a lot of these organizations exhibited such a bent well before Bolsheviks ruled Russia, or Maoists ruled China.

The parent should clarify exactly which "legitimate" governments experienced arms embargoes while trying to fight communists. If they're referring to the failure of the western powers to support the KMT, they should really look into what the KMT stood for in the 1920s. It was not the pro-capitalist, pro-colonialist movement that the western powers wanted in the region (see, for instance, the causes of the Boxer Rebellion for an illustration of what those powers did want); they explicitly called out foreign profiteering as a grievance in need of redress. Is it any wonder that those same western powers did not support a revolution which held as one of its aims the expulsion of their agents?