Relentless

Does "relentless" in the following context mean "cruel" or "comprehensive"?

Context:

In 1934, and coinciding with the rise of Nazi militarism, Dix began his crucial Flanders (1934–1936).16 In a panoramic view of no man’s land, only small, glittering patches of water and the occasional shattered tree enliven the relentless brown of the beaten-up ground. The soldiers are almost at one with the mud in which they sit.