Ah... what can I tell you?
It's pitched at 6mm, but plays fine down to 3mm or up to 15mm although I feel the abstraction falls apart beyond 10mm.
Battle groups are formed from elements, with an element being a mech, a tank, an infantry unit (somewhere between a fire team and a platoon, depending on your taste). Turn are integrated. At the start of a turn, players generate a limited supply of orders that they allocate to elements (up to two each) and then elements are activated. An active element can perform up to two orders. Command elements can generate more orders during a turn.
Damage is tracked against elements' stats, with damage degrading stats. Damage and other things also cause elements to suffer fatigue. If an element's fatigue is greater than its total stats (allowing for damage) then the element is exhausted. Not destroyed, but unlikely to play much more of a part in the battle. Elements can be destroyed, but not easily.
Players have the same access to the same list of elements and uprgrades. All these cost points. Elements have a quality that can make them cheaper (green) or more expensive (veteran or elite).
Players can also add strategic assets to their battle group. These aren't represented by miniatures, but cover things like strategic air support, tactical sub-nuclear strikes, aircraft carriers, field hospitals and black ops.
The game can be played as a straight slug-fest, but it's best played with missions and a range are offered in the book, with some missions being symmetrical and other assymetrical, some favouring one kind of army and others another.
The book includes solo play rules (as do all of my games).
The distinctive mechanic of Horizon Wars is the normal role/counter-test combo. A normal role involves rolling a number of d12s and forming them into groups that equal or exceed a target number. A counter-test involves rolling a number of d12s and comparing the results to an opponent's normal role, with results that match the normal roll cancelling those out.
12s are critical and usual result in +1 success among other factors.
Have some pretty pictures: