UniWar
https://play.google.com/store/apps/details?id=android.uniwar
Going strong since 2009
In that time I have captured over 21 thousand bases :)
My new favorite is:
Horus Heresy Legions https://play.google.com/store/apps/details?id=com.Everguild.HorusHeresy
Although it works fine as free to play, purchases have mostly cosmetic impact.
The game's universe is rich, grim and dark, hundreds of books etc BUT the game does a fanastic job of introducing the lore in an entertaing and consumable fashion.
"By the Catechism of the Autoculus of Mars! Sing Praise to the God of All Machines!"