Anarchist FAQ/What is Anarchism?/2.6
A.2.6 Why is solidarity important to anarchists?
Solidarity, or mutual aid, is a key idea of anarchism. It is the link between the individual and society, the means by which individuals can work together to meet their common interests in an environment that supports and nurtures both liberty and equality. For anarchists, mutual aid is a fundamental feature of human life, a source of both strength and happiness and a fundamental requirement for a fully human existence.
Erich Fromm, noted psychologist and socialist humanist, points out that the "human desire to experience union with others is rooted in the specific conditions of existence that characterise the human species and is one of the strongest motivations of human behaviour." 
Therefore anarchists consider the desire to form "unions" (to use Max Stirner's term) with other people to be a natural need. These unions, or associations, must be based on equality and individuality in order to be fully satisfying to those who join them -- i.e. they must be organised in an anarchist manner, i.e. voluntary, decentralised, and non-hierarchical.
Solidarity -- co-operation between individuals -- is necessary for life and is far from a denial of liberty. Solidarity, observed Errico Malatesta, "is the only environment in which Man can express his personality and achieve his optimum development and enjoy the greatest possible wellbeing." This "coming together of individuals for the wellbeing of all, and of all for the wellbeing of each," results in "the freedom of each not being limited by, but complemented -- indeed finding the necessary raison d'etre in -- the freedom of others."  In other words, solidarity and co-operation means treating each other as equals, refusing to treat others as means to an end and creating relationships which support freedom for all rather than a few dominating the many. Emma Goldman reiterated this theme, noting "what wonderful results this unique force of man's individuality has achieved when strengthened by co-operation with other individualities . . . co-operation -- as opposed to internecine strife and struggle -- has worked for the survival and evolution of the species. . . . only mutual aid and voluntary co-operation . . . can create the basis for a free individual and associational life." 
Solidarity means associating together as equals in order to satisfy our common interests and needs. Forms of association not based on solidarity (i.e. those based on inequality) will crush the individuality of those subjected to them. As Ret Marut points out, liberty needs solidarity, the recognition of common interests:
"The most noble, pure and true love of mankind is the love of oneself. I want to be free! I hope to be happy! I want to appreciate all the beauties of the world. But my freedom is secured only when all other people around me are free. I can only be happy when all other people around me are happy. I can only be joyful when all the people I see and meet look at the world with joy-filled eyes. And only then can I eat my fill with pure enjoyment when I have the secure knowledge that other people, too, can eat their fill as I do. And for that reason it is a question of my own contentment, only of my own self, when I rebel against every danger which threatens my freedom and my happiness. . ."  To practice solidarity means that we recognise, as in the slogan of Industrial Workers of the World, that "an injury to one is an injury to all." Solidarity, therefore, is the means to protect individuality and liberty and so is an expression of self-interest. As Alfie Kohn points out:
"when we think about co-operation. . . we tend to associate the concept with fuzzy-minded idealism. . . This may result from confusing co-operation with altruism. . . Structural co-operation defies the usual egoism/altruism dichotomy. It sets things up so that by helping you I am helping myself at the same time. Even if my motive initially may have been selfish, our fates now are linked. We sink or swim together. Co-operation is a shrewd and highly successful strategy - a pragmatic choice that gets things done at work and at school even more effectively than competition does. . . There is also good evidence that co-operation is more conductive to psychological health and to liking one another."  And, within a hierarchical society, solidarity is important not only because of the satisfaction it gives us, but also because it is necessary to resist those in power. Malatesta's words are relevant here:
"the oppressed masses who have never completely resigned themselves to oppress and poverty, and who . . . show themselves thirsting for justice, freedom and wellbeing, are beginning to understand that they will not be able to achieve their emancipation except by union and solidarity with all the oppressed, with the exploited everywhere in the world."  By standing together, we can increase our strength and get what we want. Eventually, by organising into groups, we can start to manage our own collective affairs together and so replace the boss once and for all. "Unions will. . . multiply the individual's means and secure his assailed property."  By acting in solidarity, we can also replace the current system with one more to our liking: "in union there is strength." 
Solidarity is thus the means by which we can obtain and ensure our own freedom. We agree to work together so that we will not have to work for another. By agreeing to share with each other we increase our options so that we may enjoy more, not less. Mutual aid is in my self-interest -- that is, I see that it is to my advantage to reach agreements with others based on mutual respect and social equality; for if I dominate someone, this means that the conditions exist which allow domination, and so in all probability I too will be dominated in turn.
As Max Stirner saw, solidarity is the means by which we ensure that our liberty is strengthened and defended from those in power who want to rule us: "Do you yourself count for nothing then?", he asks. "Are you bound to let anyone do anything he wants to you? Defend yourself and no one will touch you. If millions of people are behind you, supporting you, then you are a formidable force and you will win without difficulty." 
Solidarity, therefore, is important to anarchists because it is the means by which liberty can be created and defended against power. Solidarity is strength and a product of our nature as social beings. However, solidarity should not be confused with "herdism," which implies passively following a leader. In order to be effective, solidarity must be created by free people, co-operating together as equals. The "big WE" is not solidarity, although the desire for "herdism" is a product of our need for solidarity and union. It is a "solidarity" corrupted by hierarchical society, in which people are conditioned to blindly obey leaders.
- To Be or To Have, p.107
- Anarchy, p. 29
- Red Emma Speaks, p. 118
- Ret Marut (a.k.a. B. Traven), The BrickBurner magazine quoted by Karl S. Guthke, B. Traven: The life behind the legends, pp. 133-4
- No Contest: The Case Against Competition, p. 7
- Anarchy, p. 33
- Max Stirner, The Ego and Its Own, p. 258
- Alexander Berkman, What is Anarchism?, p. 74
- quoted in Luigi Galleani's The End of Anarchism?, p. 79 - different translation in The Ego and Its Own, p. 197