Aikido IS part of the array of arts in Japan that are considered “Martial”. The reason for this is that Martial Arts in Japan are subsumed into the practice of following “The Warriors Way”, known as Budo.
The development of “warriors”, rather than soldiers, became a necessary goal to preserve the privileges of the military class in feudal Japan after the establishment of the Tokugawa Shogunate.
Prior to the Shogunate, there was a keen interest in developing highly skilled soldiers to fight in the endless small wars fought by warlords (Daimyo) who conflicted with one another for land, natural resources, populations of “serfs” to tax, etc.
Simplistically speaking, after the establishment of the Shogunate there was less necessity for standing armies because those causes of strife fell under the dominion of the Shogun. Tokugawa consolidated his eminence using skilled warriors. He eventually codified their existence into a caste system that a person was born into, not necessarily awarded as a consequence of merit.
Approximately 5-10% of the population were Samurai. They were supported by the remaining 90-95% of the population. Since mass warfare was no longer supported those Samurai needed to serve a useful function. Many became the equivalent of policemen – protectors of their villages – or bureaucrats. The Samurai needed to maintain their skill sets and show the population their worth, however. At times during the Shogunate, the samurai had total life-and-death control over the peasant class. They needed to instill fear to maintain their preeminence They needed to show that they deserved to be on the top. Devising, practicing, and maintaining their position was furthered by demonstrating their skills in martial (warlike) training.
As the decades advanced senseless murder by the samurai was not supported; the importance of being proficient in warrior’s studies that were no longer honed by mass warfare became even more important. Different clans developed their own systems, or ryus, to enshrine their prowess and differentiate themselves from one another.
Systems dedicated to hand-to-hand combat (tai-waza) evolved into specialties such as Jujutsu and Karatejutsu. Weapon arts for the sword, staff, spear, and other weapons established themselves as Kenjutrsu, Yarijutsu, Sojutsu. Jojutsu, Bojutsu, Naginatajutsu, etc.
Those systems all required training modes that were non-lethal. Eventually, the samurai developed forms that purported to convey other values than simply fighting prowess. Budo (the warrior’s path or way), was created and enshrined as a spiritual as well as a physical endeavor. It was mandated as the underlying ethic of the Samurai class.
Purely physical forms, such as Jujutsu, often developed subsets such as Aikijujutsu.
Those forms carried over into the culture after the Miji Restoration abolished the caste system and became a “path of study” or “do”. Judo, Aikido, Kendo, Jodo, and Karate-do are all such studies.
Consequently, since Aikido is a direct descendant of Aikijujutsu and was re-envisioned during the 20th Century, is can be considered a Martial Art.