Propeller torque can be one of the most prominent left turning
tendencies. It is most pronounced when the airplane's airspeed is low
and the engine is operating at a high power setting.
American made airplanes' propellers rotate to the right from the perspective of watching it inside the cockpit. Though there are variances based on the horsepower of the engine, the size of the propeller, and the size of the airplane, the torque that the airplane's engine exerts on the propeller is very large in any case.
Because of Newton's Third Law of motion, the force that the engine exerts to rotate the propeller (the torque) is met with a reactive force that is exerted back on the engine, and therefore the entire airplane. Since the ground (which absorbs this force before flight) is absent after takeoff, this reactive force presents itself in the form of a left roll/yaw moment in flight.
(Photo: Flight Instructor Guide)