TACTICA: ALLIED DETACHMENTS
When is it a good idea to take allies? The short answer: almost always. Allies will generally improve any army you care to mention. Why?
1. Parental Shortcomings
A bit like the Emperor, really. Every army has weaknesses. This is an inescapable fact of life, and we have to minimise our army's weaknesses whilst maximising it's strength to win the game. A prime example is the Tau Empire. They have no combat units. At all. Sure, they have Kroot units, but they're a bubble wrap, not a true combat unit. A combination of low leadership across the army, fairly universal T3 on their infantry, and medium/low armour saves excepting their Battlesuit support means Tau have a glaring weakness in combat. Of course, they have mitigation; tough transports that are hard to disable, bubble-wrap, long ranged guns across the board and assault phase movement are an important factor for any Tau list and one that a player will exploit. This is common knowledge.
Despite this, a good opponent is likely to get units into combat at some point. Even with all the firepower you can muster, there's sometimes simply no way of avoiding combat. You get Supporting Fire and Markerlights, but using Markerlights to shoot dudes who are charging means you didn't use Markerlights to kill them when you were higher BS, and Support Fire is unreliable at best (as all Snap Shots inherently are). With the Allied Detachment system, however, new options are opened up. The Khorne Berzerkers reach the squishy blue people and prep for a leisurely game of Rip-the-Tau-to-Pieces-and-Eat-Him. Suddenly, a particularly angry looking Assault Terminator squad appears and hit the berzerkers over the head with big hammers. Quite hard. If combat isn't going your way, a Tau Commander gives the Terminators an I4 Hit and Run. This is very excellent.
The Tau have fragile, abysmally weak troops with no close combat weapons and unfeasibly large guns, with some fragile but strong torrent units. The Space Marines bring tough, strong people with a variety of weapons and a low number of high-strength attacks. Are they not the perfect counterpart to Kroot? In addition, they're Battle Brothers. Therefore, the low Leadership of the Tau gets ATSKNF to shore it up. Their powerful firepower gets even stronger with Null Zone. In short, Space Marines make Tau betterer.
Tau and Space Marines is an obvious one, but it's far from the only good match-up. I'll take this chance to talk about the cheap hookers of Allies, the Imperial Guard. Why ally with Imperial Guard? Firstly, because you can. Imperial Guard are up for grabs for just about anybody (Sorry, Tyranids, you have no friends). Secondly, because they have one thing that very few people have yet: anti-air. They have shit tonnes of it. Hydras, Sabres, and the jewel in the crown of anti-air capability, the Vendetta Gunship. In fact, the Vendetta alone is probably worth allying with the Imperial Guard for. Considering you need a squad of Veterans, you can bring a Vendetta-load of Meltagun-toting BS4 duders complimentary to the actual deal, the Vendetta. Any army without access to a Flyer (and some with) should definitely consider this option.
The other army that fits in with any other army is the Eldar. They're not quite as accessibly as the Imperial Guard, but by no means untouchable. They're fantastic Allies due to their Farseer's Divination, Eldar Jetbikes and their frankly astonishing number of Blessings. Run some Guardian Jetbikes (anything from one squad of 3 to two squads of 9) as Troops, and perhaps a combat unit (Scorpions) if you're playing Tau or Guard (Guardian Support Platforms if you're playing combat armies: Orks, Wolves or Tyra - wait) and you're pretty well set. For anyone who isn't a Battle Brother, a Farseer is kind of wasted - in this case, run a Solitaire (Autarch on Jetbike with Lance and Mantle of the Laughing God). For BBs, a Seer Council and Farseer are almost always worth looking at.
Imperial Guard and Space Marines: combining tough, high-quality and expensive guys with fragile, low-quality and dirt cheap guys, and getting solid air support with the Vendetta and Stormtalon.
Dark Eldar and Eldar:Wyches give the fragile Eldar a good tarpit/combat unit. Divination and Runes of Fate help Incubi, Kabalites, pretty much any unit, even moreso with the advent of the new Codex. Taking a Seer Council on Jetbikes means you can cast Conceal frequently on your Raiders and Ravagers, increasing their Jink to 3+ or even 2+, hugely offsetting their fragility, while both Conceal and Protect do the same for infantry (Protect on Kabalites to take them from 5+ to 4+ armour is pretty important, and with a bit of luck you can go to 3+ armour or better. In short, Eldar support Dark Eldar very nicely.
Orks and Imperial Guard: Orks and Imperial Guard both swarm the board, although Orks provide muscle and shitty, low-quality vehicles while the Imperial Guard give ranged power and a strong armoured presence.
Eldar and Tau:Prescience/Guide mitigates Tau BS3. Misfortune makes the Str5/AP5 of most Tau guns into a real force, especially combined with Markerlights to strip cover saves. Harlequins and Scorpions give good combat ability. If you've got money in the bank, Wraithguard/Wraithlords/Wraithknights add a very tough unit to the army. Again, psychic power is the main reason to take this matchup - Protect giving your Fire Warriors Power Armour or giving Crisis Suits 2+ saves is pretty mega.
Ultimately, taking Allies is a very very good idea. Most gaps in an army's defence can be filled under the Allies system, and less weaknesses is almost always a good thing. I'm sure I've missed stuff out (the Allies system is hugely diverse), so if you think of things, post them and I'll edit them in.
Last edited by MidnightSun; 06-15-13 at 02:56 PM.