The Crimean invasion had many reasons. First, it is an attempt to keep Ukraine as a Russian vassal state. Ukraine is essential for Russia to rebuild its empire

The Crimean invasion had many reasons. First, it is an attempt to keep Ukraine as a Russian vassal state. Ukraine is essential for Russia to rebuild its empire and restore its power. Ukraine served as a big agricultural and industrial resource for Russia for long time. Also many prominent Ukrainians made significant input into building the tzar Russian empire and later, the USSR. In order to keep Ukraine in its orbit, Russia systematically undermined Ukrainian government and security using corruption (which was extremely effective). However, when a significant number of Ukrainians became more and more convinced with democracy (and here the strong political and cultural Western influence is apparent, nothing secret about it) they finally protested against corrupted politics provided by then-president Yanukovich supported (and controlled) by V.Putin. With the occupation of Crimea and Donbass (Eastern Ukraine), Russia tries to keep influencing Ukraine in a military way, since the old way through corruption is no longer possible. It also played well internally and ensured electoral support for Putin as Russia economy started declining following the fall of oil prices. The occupation also intentionally demonstrated that the existing international law has no power it claims to have which implies that the global security system should be redesigned.