These areas had been in armed conflict since 2014 with Ukraine, and there were many casualties on both sides.
Right, its "these areas" that were in conflict with Ukraine. No Russians entered Ukraine in 2014. Okay
In April 2014, armed conflict began in eastern Ukraine between Russian-backed separatists and Ukraine. On 12 April, a fifty-man unit of pro-Russian militants seized the towns of Sloviansk and Kramatorsk. The heavily armed men were Russian Armed Forces "volunteers" under the command of former GRU colonel Igor Girkin ('Strelkov'). They had been sent from Russian-occupied Crimea and wore no insignia. Girkin said that this action sparked the Donbas War. He said "I'm the one who pulled the trigger of war. If our unit hadn't crossed the border, everything would have fizzled out"
As conflict between the separatists and the Ukrainian government escalated in May, Russia began to employ a "hybrid approach", combining disinformation tactics, irregular fighters, regular Russian troops, and conventional military support.[99][100][101] The First Battle of Donetsk Airport followed the Ukrainian presidential elections. It marked a turning point in conflict; it was the first battle between the separatists and the Ukrainian government that involved large numbers of Russian "volunteers".[102][103]: 15 According to Ukraine, at the height of the conflict in the summer of 2014, Russian paramilitaries made up between 15% and 80% of the combatants.[87] From June Russia trickled in arms, armor, and munitions.
In response to the deteriorating situation, Russia abandoned its hybrid approach, and began a conventional invasion on 25 August 2014.[114][116] On the following day, the Russian Defence Ministry said these soldiers had crossed the border "by accident".[117][118][119] According to Nikolai Mitrokhin's estimates, by mid-August 2014 during the Battle of Ilovaisk, between 20,000 and 25,000 troops were fighting in the Donbas on the separatist side, and only 40–45% were "locals"