These aren't Russian soldiers. This article is about the civil war in Ukraine. These are people with Ukrainian passports. The article uses the phrasing "Pro-Russian" because the separatists wanted economic integration with Russia after gaining independence. This is pragmatic, they would naturally be cut off from the west, and their existing economic integration with Russia was an asset to them. There's no invasion detailed in this article, other than by the AFU if you consider the break away republics to have been legitimate.