Monarchs as supreme command of the Armed Forces (and military formation)
In the European parliamentary monarchies, it is a widely observed and shared tradition that future Heads of State develop a military career and, within this framework, receive training of this nature, which varies according to the characteristics of each country, in addition to university education. This practice is based on the fact that, in most cases and under the terms and with the scope established and regulated in the different constitutional texts, they are attributed the supreme command of the Armed Forces.
United Kingdom:
Head of the Armed Forces is the position of the sovereign of the United Kingdom as commander-in-chief of the British Armed Forces. However, supreme military authority has been delegated by the monarch to the Defence Council of the United Kingdom, a body officially charged with the direction and administration of the Armed Forces.
https://en.wikipedia.org/wiki/Head_of_the_Armed_Forces
Spain:
The supreme command of the Armed Forces corresponds to the King of Spain, who acts as Captain General of the Army, the Navy and the Air Force and Space.
The king regularly chairs sessions of the National Security Council, the Joint Chiefs of staff and the individual general staffs of each branch of the Armed Forces in his capacity as supreme commander.
All promotions to military rank and positions in the high command of the armed forces are made by Royal decree signed by the king and the minister of defense.
https://en.wikipedia.org/wiki/Spanish_Armed_Forces
Denmark:
The position of the Danish monarch as the head of the military is deeply rooted in tradition. While the 1953 constitution does not explicitly designate the monarch as commander-in-chief; it is implicit, given the general provision in article 12 and the more specific wording of article 19 (2).
Norway:
The formal commander-in-chief is King Harald V; however, the de facto supreme decision-making is made by the Cabinet, led by the Prime Minister.
https://en.wikipedia.org/wiki/Norwegian_Armed_Forces
Belgium:
Article 167 of the Constitution of Belgium designates the king as the commander-in-chief. In practice, the Chief of Defence is the head and commander of the Belgian Armed Forces. He reports directly to the Minister of Defence and is responsible for advising the Minister, for the implementation of defence policy and for the administration of the department.
Netherlands:
The Constitution of the Netherlands states, in article 97, that "the Government shall have supreme authority over the armed forces". Article 42 defines the Government as the Monarch and the ministers, and that only ministers are responsible for acts of government.
Sweden:
The Monarch (as of present King Carl XVI Gustaf), is still a four-star general and admiral à la suite in the Swedish Army, Navy and Air Force and is by unwritten convention regarded as the foremost representative of the Swedish Armed Forces. The king has, as part of his court, a military staff.
Sources: Wikipédia