In orther for the country to be considered as peaceful, it needs to have a very low crime rate. The countries that I know that has really low crime rates are Abu Dhabi, Japan, Iceland and more. Also, aside from the low crime rate, there’s also the good government officials.
In the United Arab Emirates and Japan, the punishments are quite tough, and that may be the reason why the crime rates are low. Also, the conviction rates are quite high in Japan (the Japanese police claims that they have close to 99% conviction rate). I am not sure why the crime rate is low in Iceland. One of the reason may be the absence of large numbers of immigrants.