Over the course of my career I've been in a variety of business war rooms. Some of them have had only a few people and lasted only hours, and one notable war room had several full teams and lasted a full month.
Too long; Didn't read (tl;dr) - Jump to the suggestions
Generally speaking, a war room is a room from which war, business, or political strategy are directed or planned.
In business (and especially software development), it is essentially any long-running meeting designed to elevate a single priority over all others while minimizing outside influences over that priority.
I found the blog post The War Room Concept in Project Management does a good job of explaining the relationship between literal government war rooms and business war rooms.
There are many styles of war room. The most obvious is the physical war room, but a war room can also be achieved using teleconferencing tools to produce a digital war room. The key difference between a normal meeting and a war room is that the meeting is generally time-boxed to the length of time on the calendar invitation, and other priorities may override the meeting.
A war room provides just about the tightest/shortest feedback loop possible. With everyone in the same room (or on the same call), every question and statement can get a quick reply.
The war room will have a single, overriding purpose: the war. In business this is usually war upon a problem. Some examples from my experience are:
In all of these examples, the topic at hand was the single focus for the duration of the war room. Any tangential conversation would be quickly overridden and put aside for later unless it could be shown to be central to the focus of the war room.
During a war room, all members are expected to protect the war room against unrelated interruptions. Any person or communication coming into the war room not related to it are expected to be filtered out. This may sound like I'm restating "Focus", but the significant difference here is that interruptions do happen with regular meetings, and sometimes meetings are canceled for higher priority meetings. With a war room, nothing else is allowed to be higher priority, so all interruptions are denied.
When war rooms are physical and are expected to run a full day or longer companies will often have meals catered. This may seem like a small thing, but removing all conversation about what to have for lunch, where to get it, when to have lunch, and other such decisions unrelated to the war room can help things go more smoothly. It's also a good way to help calm turbulent emotions which may occur.
The high degree of focus on a single topic clobbers all other priorities regardless of importance, urgency, affected parties, or cost.
For example, the company may send out a communication stating, "All employees must complete this action by this deadline if they want to have the company benefits the next year." The war room may prevent employees from receiving that message, or prevent them from having time to complete the required action.
The war room will attempt to filter out interruptions not related to the topic at hand, but in so doing the war room causes a significant increase in the number of interruptions to individuals. What is worse, these are interruptions that are a lot more difficult to ignore, because they're relevant to whatever work the individual is doing. Many studies have shown the difficulty of doing work that involves the creative portion of the brain when there are interruptions. In a war room almost all of the interruptions to an individual will be difficult to ignore.
Due to the difficulty of ignoring war room interruptions, individual productivity will drastically decrease. A common complaint from participants of war rooms is how little they are able to accomplish. The overall target of the war room may be accomplished, but individually a lot of time is wasted. Measured in work hours this can represent a very large amount of wasted company resources depending on how many people are in the war room.
Time can be categorized along two axes: urgency and importance. Things which are both urgent and important are emergencies. They must be accomplished, and must be accomplished quickly. By definition a war room will always be an emergency.
War rooms will often include many of the features of meetings. There are many people who have documented the negatives of meetings (both physical and teleconference), so I won't include them all here.
High intensity focus on a single problem is generally stressful. Emergencies are generally stressful. Combined with other aspects of the war room, the stress level can be very high.
Sometimes people don't want to acknowledge that a war room is in fact a war room. They want to mask this for positive reasons (e.g. To improve morale), but this is a form of obfuscation, and trying to dress it up with fancy words won't fool the participants.
Catering long running war rooms is a good thing, but it can become a bad thing.
When the food is consistently unhealthy (such as fried foods or pizza for every meal), people will get upset because they physically won't feel well. Pizza is generally considered a good option for catering because it's fairly low cost per person, but repeating it every day becomes a problem.
People also desire some variety. The same food for every meal becomes a problem.
Catering also shouldn't be considered an excuse to require everyone to work through every meal. Sometimes people need a short break to arrange other things in their lives (such as rescheduling appointments).
Catering isn't a requirement, even for a long running war room, but if the war room is long running people will need a break to prepare their own food. Not catering a war room and requiring short breaks will cause more problems than it will solve.
The physical environment of the war room can also be problematic. Here are some of the problems people have expressed with physical work spaces (some depending upon the length of the war room):
It may be necessary to have people come in earlier than usual, stay late, or work on weekends. It's reasonable to ask for some of this on occasion, but if there are many war rooms in a row or long running war rooms, be careful about how often and long everyone has to work.
The war room might require that the work remain physically within the room. This could be for a number of reasons, such as requiring physical access to certain tools, requiring security, or even due to having limited access to the room itself. People will want to finish the war room as quickly as possible, and restricted hours could also be a problem.
For any, many, or all of the above negative reasons, morale will often be low for a large percentage of war room participants. War rooms can be very demoralizing.
As stated above the war room is likely to have high stress levels and be an emergency. Staying at high stress levels and in the emergency category of time is the fast track to burnout. Burnout is when someone experiences physical or mental collapse due to overwork or stress. Pay close attention to the well being of war room participants. They will be no help if they're burnt out.
War rooms can be very difficult to track and record. Generally a recording of the meeting won't suffice, because it is unlikely to be replayed due to length.
Additionally, tracking isn't the priority of the war room, so documenting the war room may be counted as unnecessary. In that case remind people of the importance of documentation.
War rooms are also generally not sub-divided into logical meetings; they are a single long meeting. This can also make them difficult to document.
You've decided you need to have a war room. You have an emergency that requires a high degree of focus and isolation from unrelated interruptions. You need to bring together people to increase communication. Here are some suggestions:
As with any meeting you should set a clear, specific agenda. If something comes up during the war room which would extend the scope, err on the side of keeping the agenda limited. As soon as the focus of the war room is achieved end the war room. Try to use the same principles you would apply to improving the quality of normal meetings.
When individuals in the war room are given a task, allow them to leave the war room to accomplish that task. If someone has a question for the person while they are outside the war room, first determine whether it is of higher priority to get the answer or to allow them to finish the assigned task. If the task is more important write down the question and save it for their return. This will increase individual productivity and morale.
You may not want to admit that it's a war room. You may not like war rooms any more than the people you're requiring to attend. Don't waste any energy trying to dress it up. Call it a war room, and then finish it quickly.
If the war room is expected to span the time people normally have meals, but you want people to work through the meal, then it's a good idea to provide food.
If you must host your war room physically somewhere you don't usually work, ask yourself how long you'll be there and what problems you see with the new space. A little prevention goes a long way.
Pay attention to the health of your team. Listen to your team's feedback, and prompt them for it if necessary. Do what you can to improve the situation based on their feedback, and when you can't address their concerns explain why. Attrition caused by a war room may not end with the war room. What is worse, a propensity toward calling war rooms will likely negatively affect the reputation of your company.
Decide ahead of time that you will track the war room, and generally how to perform that tracking. It's okay to change the approach partway through.
I personally don't like war rooms at all. I feel they are a greater loss of time and ability than the gain they provide, but I cannot deny that they have their place. If you have an emergency a war room might be needed, but if you must call for one... Please take these thoughts into consideration.