What was the mandate system, and why did it leave many groups feeling betrayed?

The mandate system is defined as it having been "a legal status for certain territories transferred from the control of one country to another following World War I, or the legal instruments that contained the internationally agreed-upon terms for administering the territory on behalf of the League." Some groups felt betrayed, because the some of the mandate contained secret clauses and amendments which benefited some countries, while others were treated unfairly due to this.