What is the ruling of Islamic law on a group of individuals building houses on lands that are not their property, some of which are occupied and undistributed lands, and some of which are lands belonging to the army? And is this considered usurpation of land?

Seizing military lands and buildings is a forbidden act because they are public property that no one is allowed to benefit from for personal gain. As for land that has not been distributed due to occupation, if it was designated for a specific purpose or is close to urban areas, it is not permissible to claim ownership of it because it is public property. However, if it is far from urban areas and not designated, it is considered fallow land, and any citizen has the right to benefit from it by being the first to utilize it.