Division is weakness

Overview
"Division is weakness" is essentially an argument that unity on some issue is more important than making the best choice, possibly on a different issue. It is one logical step away from ignorance is strength, one of the three Party slogans from George Orwell's dystopian novel Nineteen Eighty-Four, in that it views education as the source of dissent and therefore promotes ignorance as the solution to the problem of "weakness" (see, below).

Legitimate Use
A reasonable use of this argument is in a situation where people are in disagreement over some detail, or some irrelevant issue, when there is a much more pressing issue on which action is being prevented by the disagreement.

In this situation, though, a better argument would be to point out the more important issue and push for quick resolution or shelving of any minor or irrelevant issues so the more urgent issue can be dealt with. This is more likely to be persuasive, and addresses the actual meta-problem of correct prioritization rather than trying to force unity without explanation.

An exceptional legitimate use might occur in circumstances where it is more important to take some action than it is to be certain of taking the best possible action. It seems unlikely that this sort of thing would reach the point of being a public issue, however, before a decision had to be made, and hence such a decision would end up being made by someone in authority rather than in the sphere of public opinion.

When trying to decide in retrospect whether a leader made a correct call in choosing imperfect action over careful deliberation, the question should resolve around whether the imperfect action chosen was reasonable under the circumstances – rather than any claimed need for unity.

Illegitimate Uses
Most or all non-legitimate uses of this argument occur when the "more pressing issue" is actually the same issue over which there is disagreement. In this case, the argument is essentially saying "it is more important to do something than to do the right thing" – which is almost never the case in matters of public discussion. (See for further discussion of exceptions.)

Under these circumstances, this argument is essentially an attempt to justify suppression of dissent.

Equating division with weakness can also be used to argue that education is harmful, as an educated citizenry may have opinions which differ from those of their leaders (hence 1984's "Ignorance is strength"), leading to disagreement. If, on the other hand, a population is ignorant of the relevant facts, they to trust the information coming from (and decisions made by) their leaders – which makes it much easier for a leader to effectively control the population, thus greatly increasing that leader's effective (military) strength over that of other countries with less restrictive governments.

Quotes

 * "We must all hang together, or assuredly we shall all hang separately." – Benjamin Franklin (source: wikiquote)
 * "United we stand, divided we fall" – popular saying

Applicability

 * US-Iraq War: This equation is often applied to argue that dissent among Americans makes us look weak, and emboldens the enemy (i.e. the terrorists). However, what "makes us look weak" far more than this is clumsy and ineffective use of our military.
 * It has been argued that we need another 9/11 in order to rally the public behind the War on Terror and quell dissent.