In its broadest sense, the term "officer" includes non-commissioned officers and warrant officers. However, when used without further detail, the term "officer" almost always refers to commissioned officers, the more senior portion of a force who derive their authority from a commission from the head of state of a sovereign nation-state.
The proportion of officers varies greatly. Commissioned officers typically make up between an eighth and a fifth of modern armed forces personnel. In 2013, officers were the senior 17% of the British armed forces, and the senior 13.7% of the French armed forces. In 2012, officers made up about 18% of the German armed forces, and about 17.2% of the United States armed forces.
Historically, however, armed forces have generally had much lower proportions of officers. During the First World War, fewer than 5% of British soldiers were officers (partly because World War One junior officers suffered very high casualty rates). In the early twentieth century, the Spanish army had the highest proportion of officers of any European army, at 12.5%, which was at that time considered unreasonably high by many Spanish and foreign observers.
Within a nation's armed forces, armies (which are larger) tend to have a lower proportion of officers, but a higher total number of officers, while navies and air forces have higher proportions of officers, especially since aircraft are flown by officers. For example, 13.9% of British army personnel and 22.2% of the RAF personnel were officers in 2013, but the army had a larger total number of officers.
Having officers is one requirement for combatant status under the laws of war, though these officers need not have obtained an official commission or warrant. In such case, those persons holding offices of responsibility within the organization are deemed to be the officers, and the presence of these officers connotes a level of organization sufficient to designate a group as being combatant.