Ukraine did not respond because the military was in complete disrepair. For instance the army didn't even have batteries for their trucks. Also, after 75 years of Slavic brothers crap propaganda, Ukraine couldn't believe their "brothers" would shoot at them.
As said prior Ukraine has been a failed state in every sense of the word.....the only reason they have defenses now is because they are now a military welfare state....dependent on hand outs from the US and other nations.