Cross border artillery strikes
Late in 2014 reports of cross border artillery strikes on Ukrainian positions began surfacing. These reports have been analyzed by several groups, so far the most most detailed report was presented by the Bellingcat group. Based on ballistic, video, photographic and satellite evidence it was proven beyond the shadow of a reasonable doubt that Russian artillery forces attacked Ukranian forces inside Ukraines border.
The Guardia, made a very good video presentation about this topic:
Read the Bellingcat report for all the details:
https://www.bellingcat.com/news/uk-and-europe/2015/02/17/origin-of-artillery-attacks/
Corroborating evidence
Some Russian soldiers have quit the Russian army due to the Ukraine situation. One of the soldiers claimed that they were given coordinates to strike with their artillery batteries, the soldier admits that the coordinates could have been inside Ukraines territory.
http://www.reuters.com/article/2015/05/10/us-ukraine-crisis-soldiers-specialreport-idUSKBN0NV06Q20150510
Donbass separatism operation
There is evidence that the separatist movement in the Donbass was a Russian operation. The presence of Russian citizens amongst the leadership of the first separatist movement is particularly telling. For example, meet Igor 'Girkin' Strelkov a Russian citizen, he held a high ranking position within the separatist movement and for brief period was even the leader of the separatists. In Strelkov's own words "I was the one who pulled the trigger of this war" and ""If our unit hadn't crossed the [Russian-Ukranian] border, everything would have fizzled out — like in [the Ukrainian city of] Kharkiv, like in Odessa". So there you have it, an admission from one of the separatist leaders that this whole conflict was incited by non-Ukrainians and the evidence on this wiki shows that Russia is continuing to fuel this artificial separatist conflict:Crimea
TODO… kind of obvious though