Russians Furious After Ukraine HIMARS Strike ‘Kills 100 Troops' in Russia at near the border to Ukraine

If confirmed, then this is a very serious strike! Was this strike in compliance with e.g. the Geneva Convention or similar treaties? Such strikes could backfire and escalate the Russo-Ukrainian War (World War III by any other name).

"... the massacre occurred near Kreminna in Luhansk Oblast when members of the 20th Combined Arms Army gathered in a large group not far from the front lines and in range of Kyiv’s HIMARS rockets for two hours. ..."

Russians Furious After Ukraine HIMARS Strike ‘Kills 100 Troops' Ukraine has not officially confirmed the strike, but one official told Kyiv Post they ‘stood in the open air for two hours’, long enough to 'transport the HIMARS, enter the coordinates and hit them.'