Putin's reaction was a response to the shelling by Kiev of Eastern Ukraine. Ethnic Russian Women and children were being murdered. Did you expect no response?
Before which an armed uprising was started in Ukraine by heavily armed Russians, led by a former GRU colonel, who entered Ukraine, and this all occurred without the approval of the Russian government?
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".