Recreational drugs can be divided into three categories. There are stimulating substances (uppers), calming/tranquillising substances (downers), and there are psychedelic substances (psychedelics).
Tobacco is a stimulating substance, although many smokers will say it has a calming effect on them. However, this calming effect is not a characteristic of tobacco. Tobacco is, after all, a stimulating substance that makes people restless.
The calming effect is due to the fact that people have become physically dependent on tobacco. Because of habitual smoking, the body has gotten used to a certain level of nicotine. When this level goes down the body reacts with withdrawal symptoms. One of those withdrawal symptoms is restlessness. This withdrawal symptom can be temporarily suppressed by increasing the nicotine level. You do that by smoking again. The restlessness disappears, and you feel calm. This explains the calming effect of tobacco.
You think that tobacco calms you down, but you forget that the tranquility is in fact nothing more than disappeared feelings of restlessness. This restlessness is caused in the first place by tobacco. Tobacco brings you into a vicious circle. Tobacco causes restlessness which you temporarily suppress with tobacco. By smoking again, your body gets used to the nicotine, however, and your feelings of restlessness and your need for tobacco will only increase.