subject nation; subordinate state; dependency
A country that is politically or economically subordinate to another, often used in historical or geopolitical contexts.
That small country has long been treated as a subject nation of the neighboring great power.
Historically, many subordinate states fought for independence.
保護国 (protectorate) implies a protective relationship, often with some autonomy retained, whereas 隷属国 suggests a more subordinate, controlled status.
Compound of 隷属 (れいぞく, 'subordination, servitude') and 国 (こく, 'country'). The term is a straightforward Sino-Japanese construction.