U.S. drug maker Pfizer Inc agreed on Tuesday to terminate its $160 billion agreement to acquire Botox maker Allergan Plc, in a major victory to U.S. President Barack Obama's drive to stop tax-dodging corporate mergers. The decision to end the biggest tax "inversion" ever attempted, which would have seen Pfizer slash its tax bill by redomiciling to Ireland where Allergan is registered, came a day after the U.S. Treasury unveiled new rules to curb inversions.