vassal state; dependency
A state that is subordinate to a more powerful state, often in a historical or political context.
That small country was a vassal state of the neighboring great power for a long time.
Historically, many vassal states paid tribute to their suzerain.
独立国 is a country that governs itself, whereas 属国 is a dependent state controlled by another nation.
vassalization; turning into a vassal state
Compound of 属 (zoku, 'belong to, subordinate') and 国 (koku, 'country'). The reading ぞっこく is a regular on'yomi combination.