Maru is a large section in a resolute register. In the wider Indian tradition the measure carries associations with the desert, with the battlefield and with steadiness in the face of death, and the section holds the solhe, long compositions built in stanzas of sixteen, together with vars.
The standing of this measure in Sikh usage cannot be separated from the history of the community that used it. The Guru Granth Sahib was compiled in 1604 and completed in its final recension a century later, and between those dates the community was subjected to sustained persecution, lost two Gurus to execution and became an armed body. A measure associated with resolve in the face of death was not an abstraction for the people singing it.
That said, the material in the section is not martial in content. It does not celebrate battle and does not describe one. What it carries is the register: unhesitating, unadorned, addressed to a listener who may not have long. The distinction matters, because the later Sikh tradition does have a substantial body of explicitly martial composition, and almost none of it is in this volume.
Where the two are conflated, the Guru Granth Sahib gets read as a warrior's book, which it is not. It is a devotional volume, sung, that a community under threat found sufficient.