One of the most dramatic examples of solidarity in the 20th century, apart from the solidarity of populations at war, is the Polish Solidarity trade union^[1], founded in 1980 and led by Lech Wałęsa. According to Wikipedia,

Solidarity is not just a common generic noun, such as we find in a dictionary. It's a fundamental principle of Catholic social teaching and figures prominently in some important Papal documents. It appears in the Catechism in several places, and can easily be traced to Old Testament injunctions. The greatest example of solidarity is Jesus' solidarity with humanity. It's why he submitted to death on the cross - a death designed for criminals - though he himself was sinless.

Solidarity is idealized in Acts 2:44-47 and the following...

In my prior post on the principle of subsidiarity, I discussed the history of its use in the Church, identified characteristics that contrasted it from its opposite, supersidiarity, and discussed the protocol necessary for its viable implementation. The primary issue in that implementation was to characterize the need for higher level authorities to intervene in the life of a community and when it might be productive to do so.

Subsidiarity is a fancy term for a fundamental principle of governance. It is a principle that derive s from Jesus' teaching about servant leadership. It is a principle that optimizes the balance between in dividual responsibility and individual freedom and initiative. It recognizes that man is a social animal and that social groups combine naturally to support a common purpose.