Sure- here we go again;
The Ukrainians, even prior to 2014, knew that Ukraine joining with NATO- or even with the EU- was a red line for Russia. NATO also knew- the whole world knew. It was posted in the sky with fireworks- ' Thou shalt NOT join NATO, or the EU ' because the former is an existential threat to Russia ( which no sane country would allow ) and the latter will, inevitably, lead to the former. NATO promised NOT to advance eastwards yet did so nonetheless. NATO's word is shit. In short, Russia's security will always come before Ukraine's western ambitions- just as American security will always come before Cuba's pro-Russian ambitions.
So the Ukrainian elections of 2014 returned a pro-Russian president. The US/NATO, in its greed for world domination, overthrew that president by way of a coup. Bad idea. Illegal idea. Russia-threatening idea. Ukrainian pro-Russian bad, bad idea.
So there's the aggression, for starters. The anti-Russian Ukrainians rejoiced at this illegal act and proceeded to hunt and kill pro-Russian Ukrainians. There's the manifestation of that aggression and Ukraine, after the US/NATO, is to blame. I'll assume that you know what happened after the coup. Part of your memory of events might include the packing of Ukrainian trade-unionists into a hall and burning them alive.
Maybe that isn't aggression in your book- Russians saw it differently. Me too.