coup d'état

Coup d'état, or simply coup, is a sudden illegal, often violent, taking of government power, especially by (part of) an army. It is a French word:

coup = blow, stroke
de = of
état = state

Example:
1. The head of Thailand's army and newly declared leader said the military's coup d'etat was complete and promised power would be returned to the people as soon as possible.
2. Hong Kong people travelling or doing business in Thailand are reminded to beware of their personal safety following the overnight military coup.