This may vary from country to country but for the purpose of this question. Am choosing U.S. since its a country that have very wider connection with other countries of the World.
Article II Section 2 of the U.S. Constitution, the Commander in Chief clause, states that President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States.
According to the section 2 of Article 2 of the US constitution, the president shall be the commander in chief of the Army and Navy of the United States and its various forces when ever it came on the actual service for the country.
This is different for the others countries having different distribution of the powers under the different sections of the articles.
