- Ukraine was an independent country that got invaded by a neighbouring country's military
- Georgia was an independent country that got invaded by a neighbouring country's military
Russia also regularly crosses neighbouring countries' (Sweden, Finland, Alaska, etc.) borders with fighter planes, boats and submarines.
That's something all capable militaries do on a regular basis. "Accidentally", of course. The US does this, Russia does it, China does it and so on. While it's of course not nice, it's disingenuous to use it in any kind of argument.
South Ossetia was an autonomous oblast in the Georgian SSR. When the USSR disintegrated, Georgia and South Ossetia fought a war, which ended up with a truce that lasted until the Georgian invasion in 2008.