Arty Conliffe's Crossfire (CF) is an innovative set of WW2 rules that captures the ebb and flow of infantry battle. It's my favourite wargame. It's the one I play most. And it's the one that most impresses me 'intellectually'. Like all great games its basic mechanisms are simple but subtle. It's not hardware-driven and armour plays only a minor role, so it doesn’t suit wall-to-wall tankies.
It's readily adaptable to 'low tech' pre-war conflicts such as the Spanish Civil War (1936-7). But the rules have interesting concepts and mechanisms which others have adapted and taken back to WW1 or forward to Vietnam.
The innovative features are (a) no measurement of movement or range, and (b) no fixed turns. If you can see something, you can shoot at it, and a player can continue performing actions (moving, firing, rallying etc) as long as they retain the initiative, i.e. continue to suppress or kill enemy units or take some other successful action, and do not themselves suffer suppressions or kills from 'Reactive Fire'. The rules are simple and easy to learn with no book-keeping and no lookup tables, leaving the players to concentrate on tactics.
Visibility and use of terrain are key. A CF battlefield has to be packed with terrain in order to obscure line of sight (LOS) and thus reduce ranges which are otherwise unlimited, and units typically hop from one terrain feature to. The active player needs to make judgements about whether a particular action will pay off or lose them the initiative. The other player, meanwhile, still has plenty to do, constantly eyeing opportunities for Reactive Fire. With no ranges to measure, you don't need a tape, but a piece of string is useful for checking LOS.
Movement and firing can be undertaken by individual elements or groups. The best option is to have a Platoon Commander organising a 'crossfire', hence the name of the rules. Firing factors are easy to remember: three dice are thrown for a rifle element, four for a heavy-machinegun. The number of dice is reduced by one if the target is in cover or has 'hit the dirt'. 5s and 6s are hits. Three hits are a kill, two hits suppress and one hit pins. A second suppression also kills. PCs are kept busy rallying elements from pins and suppressions, but fail to rally an element and you lose the initiative.
As in Arty's Spearhead rules, basic elements have a 1.25" square base (I use 30mm) but here they represent a section/squad rather than a platoon. For Spanish Civil War (SCW) four sections form a platoon, three platoons a company, and 3-4 companies a battalion which is the largest unit you'll need. Each platoon has a platoon commander (on a half-size base) who is essential for organising fire groups and re-motivating pinned or suppressed elements.
I currently have three Crossfire projects: Spanish Civil War, Stalingrad and Normandy, all in 15mm. They each have distinct features, including a ruined cityscape for Stalingrad and bocage for Normandy.
My SCW armies are 15mm Peter Pig figures mounted 3 to an element for rifle groups, so the 'figure scale' is effectively about 1:3. This is the rules standard. But within reason the actual scale and numbers of the models doesn't matter. You can use 20mm figures at two to an element, or go down in size to 10mm or 6mm using more figures. As this is essentially an infantry game I chose 15mm.
My fourth project was going to be the French-Indochina War/Vietnam War. I did lots of palm trees, but didn't get round to the figures. I might pursue this and/or do a Japanese force to pit against my WW2 Americans, and then maybe a British force to pit against the Japanese for Burma.
Other alternatives to WW2 include the Manchurian War (1904-1905), the Balkan Wars (1914), the First World War, and the 1948 Arab-Israeli War. With extensions and house rules, CF can be pushed much further.
A new edition is in development which will reputedly improve the armour rules. In the meantime there are some simple armour rules devised by Mike Caudron available from the Files section of the Crossfire Yahoo group.
On a 4' x 4' table you need only a company a side (not much over 30 figures), so armies can be very economical. With so few figures needed and the availability of 20mm plastics, this can be a quick, cheap and fun game to get into.