Thwart and its synonyms foil and frustrate all suggest checking or defeating another's plan or preventing the achievement of a goal. Foil implies checking or defeating so as to discourage future efforts ("the police foiled the attempted robbery"), while frustrate suggests making all efforts, however vigorous or persistent, futile or ineffectual ("frustrated attempts at government reform"). Thwart usually indicates frustration caused by opposition ("the army thwarted an attempted coup").