A jackknife warning condition controller and control method notifies a driver of a potential jackknife situation while backing up a vehicle with an attached trailer. The vehicle has a front axle with steerable front wheels controlled by the driver and a rear axle with non-steerable rear wheels. The trailer has a front axle with non-steerable front wheels and a rear axle with steerable rear wheels controlled by a trailer steering controller. The jackknife controller receives an operator-controlled vehicle steering angle and a measured hitch angle. The jackknife warning condition controller determines a directional jackknife warning condition and compares the measured hitch angle with the determined directional jackknife warning condition. If the measured hitch angle satisfies the directional jackknife warning condition then a notification is sent to the driver.