https://verfassungsblog.de/waffenlieferungen-an-die-ukraine-als-ausdruck-eines-wertebasierten-volkerrechts/
Law of neutrality. The Hague Conventions of 1907 oblige a neutral state not to participate in armed conflicts - neither with troops nor with weapons. It may also not join a military alliance such as NATO.
It prohibits neutrals from supporting belligerent states with troops or supplying them with weapons.