Reinforcements came up as the "Card of the Day" in the LotR Discord and I spent way, way too long going on and on about my love of this card-- it's one of my two favorites in the game (with Stand and Fight). And then it occurred to me that I'd never posted a review of it, so I'm cleaning it up and copying much of it over here.
From a design standpoint, the unique thing about Reinforcements (and Stand and Fight / Reforged) is the ability to use others' decks as an extension of your own. Can't fit the cards you need? That's fine, just use someone else's cards instead. Unless your partner is running Forth, The Three Hunters!, they almost certainly have allies.
It's easy to think of Reinforcements as just a double Sneak Attack, but the ability to put those allies in the "wrong" play area is crucial to the "straightforward" uses for Reinforcements, which is often bringing in two allies during the combat phase to solve any issues that need solving. Unlike Sneak Attack, which puts the ally in your play area, Reinforcements can show up anywhere-- as a practical matter, it's akin to giving any allies both ranged and sentinel, which (as anyone who has played a lot of multiplayer games can attest) is fantastic. If there's any player at the table with one more engaged enemy than ready defender, or if there's any enemy at the table with one more HP than available point of attack, Reinforcements can ride to the rescue.
If you don't need the actions for combat, Reinforcements is perfect fodder for A Very Good Tale-- getting you up to 10 (or even 11) resources worth of permanent allies at the same fixed 3 cost. (Sneak attack also works well, but Reinforcements' ability to pull the two most expensive allies from anyone's hand and to supply both exhausts for the Tale especially stand out.)
Beyond the basic case of getting extra stats and actions, Reinforcements (like Sneak Attack) is usually used as a way to repeatedly benefit from effects that trigger on characters entering or leaving play. Played during the quest phase, it can get Prince Imrahil ready for combat (or if you Reinforce Éomund, it can ready all Rohan characters at the entire table!). Éomer gets his attack boost. You can use it to tuck a couple eagles under Eagles of the Misty Mountains. You can even use it to create a trigger for Valiant Sacrifice.
Enters play effects are even stronger. Obviously something like Gandalf will be the gold standard-- Sneak Attack + Gandalf is the grandaddy of all combos for a reason. Early in the game's life, the primary problem with it was simply getting both pieces in your hand at the same time. But with Reinforcements, even if you didn't draw your own Gandalf, odds were great that at any given moment someone at the table had a Gandalf kicking around. As the cardpool has expanded, Gandalf is no longer an auto-include and these odds have declined, but Reinforcements remains as relevant as ever (if not more so), because it allows you to ask not just "does anyone have a Gandalf?", but also "does anyone NEED a Gandalf?"-- and someone always needs a Gandalf.
With a Gandalf in hand, Reinforcements suddenly becomes a The Galadhrim's Greeting (drop any player's threat by 5) or a Lórien's Wealth (any player may draw three cards)-- with two temporary allies for good measure. (I don't mention the direct damage option here both because that option doesn't depend on whose play area Gandalf winds up in, and also because there's still no real analog to Gandalf's direct damage option. The only other cards that guarantee you at least 3 points of damage are Forest Patrol and Fierce Defense. The only other cards that let you pick any enemy at the table as a target-- regardless of location, trait, or status-- are Ranger of the North and Taking Initiative. All of these cards have restrictions that prevent them from matching Gandalf's predictability and universality.)
But when you look past Gandalf and start looking at Reinforcements as a way to trigger "Enters Play" effects on allies, Reinforcements itself starts to look a lot less like an effect and a lot more like an engine-- it's not something that you want to do, it's something that enables what you want to do. What kinds of things will it enable you to do?
- Draw (e.g. Gandalf, Elrond, Lindir, Sarn Ford Sentry)
- Threat reduction (e.g. Gandalf)
- Direct damage (Gandalf, Faramir, Longbeard Orc Slayer)
- Healing (Elrond, Galadhrim Healer)
- Condition removal (Elrond, Miner of the Iron Hills)
- Location control (Meneldor)
- Engagement control (Mablung, Faramir, Guardian of Ithilien)
- Action advantage (Eomund, Greenwood Defender, Greenwood Archer)
- Stat buffs (Merry)
- Scrying (Celduin Traveler, Ithilien Lookout, Cautious Halfling)
- Search/shuffle (Bob, Bilbo Baggins, Galadhrim Minstrel, Long Lake Fisherman, Wolf/Fang/Grip)
- Resource smoothing (Envoy of Pelargir, Halfast Gamgee, Wolf/Fang/Grip)
- Incredible nonsense cheese shenanigans (Saruman)
- Recursion (Ered Nimrais Prospector, Galadhrim Weaver, Gwaihir-- this category is extra special because it's so rare and limited)
Many of those effects are the best in their class in the entire game. Nothing can beat Elrond's ability to heal an unlimited amount of damage off of a single hero, and Meneldor is essentially the location equivalent of Gandalf's direct damage, only better (since you can split the tokens to prevent overkill)-- he's like two Asfaloth and you can drop him multiple times per round. Lots of cards let you engage enemies out of turn, but-- paired with Reinforcements-- Mablung lets you move any eligible enemy to any player's play area or even back to staging.
Other of these effects are less impactful, but since Reinforcements brings in two allies, as long as one of the two is one of the high-impact stars, that more than justifies the cost and any second effect you get is a nice bonus. And given the sheer number of allies with Enters Play effects or Leaves Play effects, again, the odds that someone at the table has something useful are quite high.
The recursion category deserves its own special mention, however, because it is so rare and so potentially game-breaking. Most of the broken infinite loops in the game's life have depended on some sort of recursion, and much of that recursion has been errata'd as a result. Letting you play powerful effects round after round after round is pretty good, it turns out. Most of those powerful events require some sort of outside recursion to keep them flowing, but Reinforcements is nearly alone (exception: Host of Galadhrim) in its ability to recur itself-- simply reinforce a Galadhrim Weaver to shuffle the last copy of Reinforcements you played back into your deck. With one of the simplest recursion loops in the game, it's trivially easy to play at least one Reinforcements per round indefinitely, and not too tricky to even get a second (or, if you really want to ramp it up, third or fourth) play. If one Gandalf per round is good, four Gandalfs per round must be really, really good.
But more than that, again, the ability to put allies into play under anyone's control lets you recur other decks' key cards. In a dedicated fellowship, a single Reinforcements deck can handle all recursion for the entire table, turning on unlimited plays of high-value cards like Thicket of Spears or Doom Hangs Still or Path of Need.
Are there any other points worth making? I haven't touched on its interaction with Sword-thain yet. If you Sneak Attack in a unique ally and then play Sword-thain on it, it doesn't bounce back to your hand at the end of the phase (because Sneak Attack directs you to return the chosen ally to your hand and the card is no longer an ally). Reinforcements can do the same trick, except you can give that extra hero to any other deck at the table. Forth, The Three Hunters! decks can't include any ally cards... but that doesn't mean they can't play across the table from a Reinforcements deck and have it drop some allies in their play area and turn them into an extra hero (who gets all of the boosts of the Three Hunters contract).
Perhaps my favorite thing about it is that while many power cards can make other decks at the table feel less powerful (I'm sure most people have had the experience of playing across from a Vilya deck and feeling a bit like a sidekick), Reinforcements makes other decks feel more powerful. It makes Rohan feel Rohannier, it beefs up Eagles, it creates a Fourth the Three Hunters, it helps out with combat, it provides draw and threat reduction as needed. It's a powerful card, but also a collaborative card, one that captures well the spirit of this cooperative game we all love.
Whether you view it as useful combat smoothing, a way to get extra enters/leaves play effects, a powerful and flexible engine, or a way to engage in some fairly unique shenanigans, there's no card in the game like Reinforcements, and that's why it's my favorite card.
Well, except maybe for Stand and Fight.