Despite having a monopoly to conduct foreign affairs, the King does not have the monopoly to make war. Such a decision, it was realised centuries ago, should not be only in the hands of the King. Accordingly, any entry into war must be approved by both the Royal Council, and the Royal Court.
Military operations both during war and peacetime have been the subject of great interest with the National Royal Council. The Council has often felt that it didn't have enough control over the military. As a result, the Council has been a fervent advocate for strict rules of engagement. It was also keen to ensure that soldiers would operate with a high degree of ethics on the ground.
Accordingly in 2230, it passed the Ethical Code of Warfare which has since guided all military operations. The code governs the responsibility of the military to protect the weak, to answer distress calls, to render aid to any vessel, regulates the use of torture, etc. Two years later, the Rules of Engagement for Soldiers Serving under His Majesty's Forces further limited the Rosebourg military's possibilities to use disproportionate force, some of the interesting elements of these rules of engagement, for example, required starship captains to attempt to disable rather than destroy ships whilst engaged in combat. These have been amended over time, inspired in part by the Terran Geneva Convention.
The Monarchy is also a signatory of the O.I.P Conventions of War.
Rosebourg military forces are thus amongst the most legally-bound of forces in the Known Universe. This has not stopped the Rosebourg Monarchy sitting on the defence for accusations of its troops acting in violation of both its own laws as well as the OIP Conventions.