free to download at Wizards. Having read the article a couple times over, but not actually playtested it, I get a feeling this item is still highly experimental and that's why it's been dangled out there early on for the public to peruse. Of course this is the stated goal of the UA articles and so I'm giving the designers a pass on creativity and polish. The game is intended for mass combat using the exact same 5E rules you would for normal encounters (AC, hp, attack, damage, and so on) except scaled up in groups , or"stands" of roughly ten creatures.
The scale chosen for this new "Battlesystem" is key because it incorporates skirmishing units and solo hero involvement. Why? Because it makes sense within the logic of fantasy RPGs that one fighter for example could go toe to toe with ten lesser orcs or one big monster could threaten a small army. Stands are combined into larger units but in general still keep their average stats. Movement and spells are similarly scaled to account for the size of a battlefield. Heroes are given options to lead forces and give orders.
Terrain is handled in a rather easy fashion and mission objectives are touched on as well to give opponents a scoring system in order to see how decisive a victory ends up. The most important addition to these scaled 5E rules is the Check Morale feature which can make or break a battle. Of course solo heroes can try to step in and rally those broken units. All these rules suggestions have the making for good skirmishing battles. I would also hope there's some way to better represent sieges in the rules at a future time.
My take on When Armies Clash? Compared to the old AD&D Battlesystem it's a good start (I still possess all my counters and use them endlessly) though I highly doubt I'll use it in this current form. I wonder how much magic can break this game since 5E spells seem to be frontloaded for damage at lower levels now. I'm also mainly concerned how well 5E rules hold up as you increase the size of regimented units. A small army led by play characters against the Horde of Elemental Evil is fine by these rules. However, I'm biased to large epic battles when it comes to my Greyhawk campaigns since Gygax was originally a wargamer and set up things for conflicts on a continental scale. There is easily more than a dozen battles in Greyhawk canon that involve thousands of soldiers per side. Never mind historical examples of mass combat, a typical trope in fantasy warfare is the vast enemy horde which always seems to be 10,000 strong. That translates to 1000 "stands" in this system. Now whether you use minis or counters, that's rather difficult to present and I'm sure heroes might want to stick to their regiments instead of going it solo.
For this reason my mass combat system of choice has always been the less flashy War Machine rules from the OD&D Companion Set or the card-based but cleverly organized mass combat rules from Birthright. Sure you could scale up stands to 20, 50, 100 men but then you lose the game balance for solo hero or monster combat. Basically it comes down to the story your DM is trying to tell and how much time you want to invest in a major battle. Of course what do I know? I play Warhammer 40k! If anyone tries these rules let me know!