The rules for placing models in cohesion seem quite clear to me, but the rulebook seems to imply that it is always possible to somehow place all non-unit-leader minis in a legal way.
In another context it states that the entire unit is defeated if the unit leader leaves the battlefield.

However, there are ways to force the opponent in situations where he can place the unit leader, but cannot place all other minis, for example:

Example 1
3 AT-RTs charge an enemy trooper unit leader and displace some enemy minis of the unit in the process, but not the unit leader.
After the last AT-RT has finished its move, the unit leader is boxed in in a triangle. The remaining minis of the unit can not be placed within a legal speed-1 move to the unit leader, because ground vehicles are impassable terrain to troopers.
Are the displaced minis defeated?
What happens to non-trooper units that have not been displaced by the last charge?
What happens if one AT-RT is not placed in base contact and the trooper unit decides to move, so any remaining minis have to be placed in cohesion to the unit leader but cannot?
What happens if the unit gets a panic token and the unit leader can move a few mm to the next board edge, but cannot reach it? Can he remain stationary?

Example 2

Player A outactivates player B by two or three activations. At the end of his turn, he moves two full trooper units in a double semicyrcle next to an impassable wall. Then he uses a commander with forcepush to move a near enemy unit leader of a full trooper squad right into the middle of that semicyrcle but not in base contact. Only one or two more minis of that enemy unit can be placed in cohesion.
Is the rest of the unit defeated?

May need to contact Alex Davy or Luke Eddy for a correct answer.

My guess which is sourced from pg 20 of RR: ( where from what I remember is the only place it mentions what happens if cannot be placed legally and maintain cohesion )

Quote

When a unit climbs or clambers, the bases of the minis in the unit can partially overhang ledges, as long as each mini is stable and is not precariously placed. If there is not room to place a unit leader’s base on the surface that the unit is climbing or clambering onto, or if any mini’s base cannot be placed legally and maintain cohesion, that unit cannot perform that climb or clamber action.

... would be maybe you couldn't move in a way that would displace the unit that cannot be legally placed in cohesion. That would make sense since there are ways where you could displace the entire unit including the leader where not a single unit could be placed at all, would be a fluke though. Only other thing I could think of is maybe they would be relocated as close as possible to where they were. Overall I doubt they would be defined defeated and removed from table just because of that, yet I do not know.. Alex and Luke do though.

Ultimately, claim what happens, so will your opponent... don't agree?... then...

pg 4 of RR

Quote

Players should always attempt to come to an agreement regarding disputes about situations on the battlefield. If players cannot come to an agreement, such as determining the range between two miniatures or line of sight from one mini to another, the player with the round counter should roll a red defense die; on a block (󲉣) result, that player’s interpretation of the situation is considered correct and play continues. On any other result, the interpretation of the player without the round counter is considered correct and play continues.

It's crazy that I am just realizing that this has never come up before.

Technically, by basing the unit leader, in base contact with the AT-RT is a legal placement. Infact, all minis of the unit would need to be placed in base contact with the AT-RT, it has started a melee with the trooper unit. They don't have to maintain cohesion for this placement. Only if they cannot be placed in base contact are they placed in cohesion.

Also, AT-RTs cannot overlap each other on movement, so good luck getting 3 of them to perfectly surround an individual trooper unit leader.

6 minutes ago, rowdyoctopus said:

Technically  , by basing the unit leader, in base contact with the AT-RT is a legal placement. Infact, all minis of the unit  would need to be p  laced in  base contact with the AT-RT, it has started a melee with    the trooper unit.

I think this rule only applies between 2 trooper units. Ground Vehicles are not forcing troopers into melee (although they can melee attack).

6 minutes ago, Staelwulf said:

I think this rule only applies between 2 trooper units. Ground Vehicles are not forcing troopers into melee (although they can melee attack).

Two units in base contact are in a melee.

I was wrong about the cohesion bit (cohesion is supposed to be maintained even when placing in base contact), but in this case I would say in base contact with the AT-RT is good enough. Minis don't get auto-defeated.

The whole section on melee is only about units with melee weapons in base contact with other units.

Troopers can only be engaged with other troopers, but anything with a melee weapon can start a melee.

27 minutes ago, rowdyoctopus said:

Technically, by basing the unit leader, in base contact with the AT-RT is a legal placement. Infact, all minis of the unit would need to be placed in base contact with the AT-RT, it has started a melee with the trooper unit. They don't have to maintain cohesion for this placement. Only if they cannot be placed in base contact are they placed in cohesion.

Also, AT-RTs cannot overlap each other on movement, so good luck getting 3 of them to perfectly surround an individual trooper unit leader.

You are wrong about the melee rules.

The melee rules at page 41 say the minis still have to maintain cohesion if you place them in base contact with the charging unit.

I red all relevant parts of that situation several times, because I had that situation in a game.

The situation occured more than once to me now.
It is actually very easy to set that up with AT-RTs if you have just a little bit of experience.
If all three of them move in a parallel line towards their target unit, you just have to move the first two of them to a place that is about one or two inches left/right to the target unit leader and three to four Inches in front of him (just a little bit further than the joint of the movement template).
With their second move, you move past him and make a partial move to the back of his base.
The last one then closes the triangle at the front of his base.

Very easy actually.

I just used it to keep units in place and stack 6 suppression tokens on them by now, but it would definetly better if there would be some clear rule how to handle that situation.

I would assume that with the new tanks it might be even easier to get into such situations.

They are massive, fast, can displace with pivots and have passengers that can deny more space with their disembark move.

1 minute ago, M.Mustermann said:

You are wrong about the melee rules.

The melee rules at page 41 say the minis still have to maintain cohesion if you place them in base contact with the charging unit.

I red all relevant parts of that situation several times, because I had that situation in a game.

The situation occured more than once to me now.
It is actually very easy to set that up with AT-RTs if you have just a little bit of experience.
If all three of them move in a parallel line towards their target unit, you just have to move the first two of them to a place that is about one or two inches left/right to the target unit leader and three to four Inches in front of him (just a little bit further than the joint of the movement template).
With their second move, you move past him and make a partial move to the back of his base.
The last one then closes the triangle at the front of his base.

Very easy actually.

The only part I was wrong about was completely ignoring cohesion, which I already pointed out.

Use more terrain. I can't remember the last time I played on a table that allowed 3 AT-RTs to move freely side by side. Additionally, your opponent is just sitting there with a trooper unit while your AT-RTs advance on it? Or why were you just sitting there while your opponent just rolled up with 3 RTs?

The last bullet point on page 41 makes it pretty clear that being in base contact overrides cohesion in regards to melee. This will only really ever matter with vehicles, anyway.

Sorry, I started the reply before your post was published

The ATRTs can move side by side or in a triangle, that doesn't matter. If you set them up with the intension to move up the table, you usually find an avenue of approach.

I usually run them naked as bulletcatcher, roadblock, distraction and cheap activation. Without hardpoint upgrade, they are not very high on the target priority and have 3 x2 activations + 18 armored wounds for 165 points. That is enough to keep a lot of units busy all game long.
With 2 repair droids, they usually dont take any damage in turn 1.
If the opponent moves one unit out of is deployment zone, they can potentially start to displace in turn 2.

That is just one of several options though. But it definetly happens sometimes. It depends on the opponents list and the mission what I do with them.

If they aim/shoot, they deal 1 crit on range three, which is ok against sniper strike teams. And they provide a huge area of impassable terrain.

If they lock an enemy unitleader or commander, each can aim/attack with 3 red dice, crit to surge and Impact one every turn they survive.

If you just move/reverse they can deal between 2 and 6 suppression tokens each to the opponent, depending on how many units they displace.

All the time the rest of my army can play the objective.

They are quite ok if you get experienced with them.

33 minutes ago, M.Mustermann said:

If they lock an enemy unitleader or commander, each can aim/attack with 3 red dice, crit to surge and Impact one every turn they survive.

What do you mean by "lock?"

4 hours ago, arnoldrew said:

What do you mean by "lock?"

I think they mean "engage" or "lock in melee" but if I recall correctly, infantry aren't locked in melee with vehicles.

As an aside, I too have gotten some decent usage out of kicky AT-RTs in the past.

1 hour ago, Caimheul1313 said:

I think they mean "engage" or "lock in melee" but if I recall correctly, infantry aren't locked in melee with vehicles.

As an aside, I too have gotten some decent usage out of kicky AT-RTs in the past.

No, they aren’t. Only troopers Engage.

Well... I actually meant the situation we where discussing about where 3 AT RTs are in base contact with a unit leader and he cannot move anymore because they are impassable terrain.

Rowdyoctopus claimed that the situation would not occur if I played the game correctly use, a table with normal terrain loadout and against an opponent with a functioning brain. Since the situation did actually occur, I just gave some examples how it can evolve within a normal game.

I imagine that moving the last AT-RT in to displace the unit would not be a valid move. I.e. you would have to back the walker up until the troopers could still be placed in cohesion. As far as the game state after that... well, that unit of troopers is going to die real quick anyway so it's not too important.

To add another point to the related naked AtRt topic: I think controlling the movement of enemy units is one of the main uses for them, because it is one of the things trooper units can not do.

The massive footprint and them beeing impassable is very useful in missions like recover the supplies where the player who controlls the centre until mid-game has a huge advantage. Intercept transmissions is basically the same, because tournament games often end before turn 6.

If they approach behind terrain like huts and walls or so they sometimes even get heavy cover. Even Stormtrooper lists with lots of DLTs have difficulties to take them down then, especially with the support of one or two repair droids.

14 minutes ago, KommanderKeldoth said:

I imagine that moving the last AT-RT in to displace the unit would not be a valid move. I.e. you would have to back the walker up until the troopers could still be placed in cohesion. As far as the game state after that... well, that unit of troopers is going to die real quick anyway so it's not too important.

That is true. The main purpose of the walkers is not to cause damage though. Other parts of the lists can do that. The damage they cause is just a bonus.

Their main purpose is area/movement control, distraction and objective play. If you can stop the movement of trooper units just before they reach the centre, that is often really beneficial.

They also have a lot of utility against a lot of units that rely on movement, like snowtroopers, royal guard, wookies and so on.

They are not good in every given objective, deployment or against every opponent. But even then, they are still a cheap, expendable and relativly durable activation.