Mass Effect & Mass Effect 2
The Conventions regulate the use of Weapons of Mass Destruction. A WMD causes environmental alteration to a world. A bomb that produces a large crater is not considered a WMD; a bomb that cases a 'nuclear winter' is.
Use of WMD is forbidden on 'garden' worlds like Earth, with ecospheres that can readily support a population. If a habitable world is destroyed, it will not be replaced for millions of years. The Conventions do not forbid the use of WMD on hostile or in sealed space-station environments. Many militaries continue to develop and maintain stockpiles.
The Conventions graded Weapons of Mass Destruction into tiers of concern; Tier I is the greatest threat to galactic peace.
TIER I: Large kinetic impactors, such as asteroid drops or de-orbiting space stations. Effectively free and available in any system (in the form of debris left over from planetary accretion), kinetic impactors are the weapon of choice for terrorists and 'third galaxy' nations.
TIER II: Uncontrolled self-replicating weapons, such as nanotechnology, viral or bacteriological organisms, 'Von Neumann devices', and destructive computer viruses. These weapons can lie dormant for millennia, waiting for a careless visitor to carry them on to another world.
TIER III: Large energy-burst weapons, such as nuclear or antimatter warheads.
TIER IV: Alien species deliberately introduced to crowd out native forms necessary for the health of an ecosystem. Ecological tampering can take years to bear fruit, making it difficult to prove.