The secession was poorly planned by tshombe. There was hasty proclamation on July 11 1960.
The state of Katanga was not recognised by the UNO and most independent African states.
They condemned Katanga and imposed diplomatic and economic sanctions.
The Katangese gendarmes were not well trained like the UN forces.
The UN forces successfully blocked Katanga making it difficult for Katanga to receive mercenaries and arms for reinforcement, from Portugal, South Africa etc.