This is a picture of Barth in his army uniform. During the early period of World War II, Barth, who was then 54 years old, voluntarily enlisted himself to serve in the Swiss army.
[Barth] was sent with 'the armed emergency service' to a unit which, in case of an attack upon Switzerland, had the task of holding up the German army within the border areas for a while until the Swiss regular army could gather in the 'stronghold' of the Alpine fortress." The unit, as its members knew, would hardly have had a chance of survival.
(Frank Jehle, Ever Against the Stream: The Politics of Karl Barth, 1906-1968, trans. Richard and Martha Burnett [USA: Wm. B. Eerdmans, 2002], p.69. Emphasis added.)
If the Germans had attacked the Swiss border, the Church Dogmatics will not come in 14 volumes.
It is worth noting that Barth's enlistment into the army was not so much motivated by nationalism in the service of his country. Rather, it was his resistance against the evil of the Third Reich. His enlistment was a service to God.
In his letter to Bishop Bell of Chichester dated June 19, 1942, Barth enclosed a photo of himself in army uniform with the written words: "Resist the evil with all means." (Ibid, emphasis original.)
Barth has made himself a good example of Christian engagement with politics. We participate in a country's political process not due to nation-state nationalism. Our only allegiance is to God. And our only citizenship belongs to the heavenly Kingdom, the true State to which every other temporal country is but a reflection. As Barth wrote in 1938,
It must be emphasized, above all else, that in this future city in which Christians have their citizenship here and now (without yet being able to inhabit it), we are concerned not with an ideal but with a real State—yes, with the only real State; not with an imaginary one but with the only one that truly exists. And it is the fact that Christians have their citizenship in this, the real State, that makes them strangers and sojourners within the State, or within the States of this age and this world.” (Karl Barth, Community, State and the Church: Three Essays [USA: Doubleday, 1960], p.123.)