Step 1: State who introduced it and when:
Lord Wellesley introduced the Subsidiary Alliance system starting in 1798 to bring Indian states under indirect British control.
Step 2: State the core obligations:
A subsidiary state had to keep a British garrison (paid by the state), host a British Resident, give up its own foreign relations, and often disband/limit its own army.
Step 3: State the consequence:
Failure to pay led to territorial annexation, and over time many states -- Hyderabad, Awadh, the Marathas -- effectively lost independent sovereignty under this arrangement.
Final Answer:
\[ \boxed{\text{A treaty system by which Indian states accepted British troops and lost control of foreign policy in exchange for protection}} \]