Strategy games on iOS are ten a penny, and many of them are clothed in the colours of the past. But very few actually make the effort to put you squarely in the setting, and most that do can be off-puttingly complex.
Frontline: Road to Moscow tries to walk the narrow line between accessibility, fun, and depth - and for the most part it succeeds.
It owes a great deal to PC indie strategy hit Unity of Command, but while they look similar on the surface they feel quite different to play. Frontline is easier to get into, but shallower - and that, perhaps oddly, makes it a very good fit for mobile.
The game puts you in the shoes of the German high command, orchestrating the 1941 invasion of Russia. Over a series of scenarios you'll inch closer to the Soviet heartlands, dealing with increasingly large and well-trained formations of Russian troops as you go.
The key to the system is that you're given a pool of resources for each scenario, which you can use to buy new units or resupply your existing ones as you see fit.
This provides a lot of tactical flexibility: there's a diverse roster of troops to purchase, and new units are ready for action right away, but usually start far from the front line. Resupply, on the other hand, puts a unit out of action for a turn but preserves its hard-won experience.
But that's just the tip of a deliciously diverse iceberg of strategic choice. Units differ in their capacity for movement, and their ability to fight against armour or infantry, and they have unique special abilities too.
Paratroopers, for instance, are pretty mobile and have a handy "snipe" ability to engage units at long range, but they suffer against tanks and armoured cars.
In most wargames, a fair amount of complexity is required to model this sort of diversity with any kind of accuracy. And while Frontline fails a bit in terms of authenticity (sniping paratroopers? Really?), it compensates with a superbly streamlined interface.
Tap a unit to see its movement range, and any units in firing range are highlighted together with likely combat results. That's it - everything required in easy reach.
The same simple interface works when used in various types of battles, both on land and in the air, and whether the focus is on infantry or armour.
The result is slickly thrilling, the ease of use drawing you into a game that starts out as a rollercoaster romp across the Russian steppe but rapidly ups the challenge with bigger and more determined resistance, much like the history itself.
By the first time you're liable to fail, it'll be too late: the game has its hooks in you and you'll be puzzling over how to crack the Russian lines in tougher scenarios. There are no horrible difficulty spikes, although those craving greater adversity can up the difficulty level.
Given that the game is so brilliantly easy to pick up and play, it's unfortunate that Slitherine hasn't capitalised on that accessibility with a better tutorial and, well, any manual at all. You can pick up most of what you need from the loading screen hints, but it just feels like a bizarre omission.
Another unfortunate feature to have gone AWOL is a two-player mode. Since each side's turn is self-contained and the engine has an inbuilt points-based way for each side to select a squad, Frontline: Road to Moscow seems well-suited to asynchronous play, but there's no option. That's a shame, as it would have extended the already considerable shelf-life of the game.
Frontline has some minor shortcomings, but it's one of the best, most accessible and most engrossing historical strategy games available on iOS.
There are deeper titles, but they're far harder to get to grips with. Battle Academy does beat it in most departments, but it costs ten times as much. Anyone even vaguely curious about gaming military history on mobile should snap this brilliant bargain up right away.