Inertia is the tendency of any object to maintain what?

Newton First Law of motion states that "A body continues in its state of rest or of uniform motion in a straight line until/unless an external force is applied upon it" Why is it so? "Inertia is the property of a body to resist any change in its state of rest or of uniform motion in a straight line" Newton's first law of motion is also therefore known as law of inertia because of the inertia the body will remain in its initial state forever. And mass is the measure of the inertia of a body.