Among many justification for socialism, this one actually make sense. The ming and the sung in china also fall due to wealth disparity.
The poor, which has no land to defend anyway, don’t bother fighting. Government then perform a belated response of redistributing land and confiscating property for “national defense”. This results in the rich switching side to mongols.
However, most socialists measure simply worsen poverty. Socialists take money from workers via income tax, while giving it to aristocrats and land owners via bribes, infrastructure building, and corruption.