A guild member can station troops in another member's village to help defend it. It's the most direct way to protect an ally who's about to take an attack — or who's offline.

A reinforcement never changes hands: the troops stay yours, they only defend the host village, and only you can call them back.

How to send

Open the Allies tab on the Guild screen. It lists the villages of every member, even those outside your field of view on the map.
Pick the ally's village, your home village and the troops. Heroes don't go as reinforcements, and caravans don't garrison.
For your own villages, use the realm's regular reinforcement — this tab is for allies only.
Once on its way, a reinforcement can't be cancelled: the march continues until it reaches the ally's village. Only after it's stationed can you recall it, and then the troops march back.
The March of the Order path, from the Order Tree, makes reinforcements arrive faster.

If the Allies tab doesn't show the sending form, reinforcement between members is disabled on this server; whatever is already en route or stationed is still listed: what has already arrived can be recalled, and what's still en route continues until it arrives.

Who can reinforce whom

Both sides need at least 3 days in their current guild. That clock counts from your first time joining that guild and doesn't reset if you leave and come back to it.
Neither side can be under novice protection or a Peace Pact: a protected village can't be attacked, so it neither sends nor receives reinforcements.
Mutual reinforcements that would close a loop between members are refused.
Every refusal tells you the reason on screen.

How much fits and how long it stays

Each village hosts a cap of allied troops: 200, plus 150 for each combined level of its Barracks and Wall. The cap applies when sending and again on arrival: if, when the reinforcement gets there, the village can no longer hold the troops, the entire march goes back home, not just the excess.
The reinforcement returns on its own after 1 day. You can recall it earlier, at any time: the troops march back.
Troops you receive can't be passed on to a third player.

In battle

Reinforcements join the village's defense against players and against raiding bands, together with the local garrison. They never attack.
Losses are split between the local garrison and each reinforcement, and come out of the lender's army. The lender is notified when they lose troops.
The Guild Wall bonus from the tree strengthens the defense of anyone attacked at home.

In the report

When the attacker wins, their report splits enemy losses into Local garrison and Reinforcements, without saying whose each reinforcement is. On a loss, and in the defender's report, that split doesn't show.
Scouting shows that an allied reinforcement is stationed, but not what it's made of.
Reinforcements count toward the strength the map shows for the village, but not toward the host's ranking or progression.

When the bond ends

If you or the ally leave the guild, get expelled or the guild is disbanded, all reinforcements between you return at once, in both directions — what you lent and what was lent to you. The bond is with the guild at the time of sending: switching guilds also sends everything back.

Keep track of it all in the Allies tab: Troops on loan shows what's en route to or stationed with allies, and Reinforcements received shows who is defending you and until when.

Guild reinforcements