Among Karl Barth's greatest enemies were the American and Dutch Calvinists (especially the American Dutch Calvinist Cornelius Van Til and his minions). These 20th century Calvinists were different than the neo-Calvinism popular today because they proudly called themselves Fundamentalists and wished to regress the church and theology back to 16th century Calvinism (not only the T.U.L.I.P.). It's a sad story, because Barth was also a Calvinist (or arguably a post-Calvinist) and deeply admired John Calvin and reconstructed Calvin's theology throughout his Church Dogmatics and other writings. (These self-identified Fundamentalist Calvinists disparaged Barth as a "modernist" which is ironic because they were committed to apologetics and modernistic rationalism.)
On one occasion, in the preface of the Church Dogmatics III/4, Barth had had enough of the unrelenting, scathing and slanderous attacks by these Fundamentalist Calvinists that he anathematized all of these Dutch Calvinists (who were the most vocal Fundamentalist Calvinists and closest in proximity to Barth at that time). Barth's response is too strong but the Fundamentalists Calvinists had it coming because they had accused Karl Barth of being the greatest heretic of all time and constantly made all kinds of unsubstantiated accusations, and even went so far as to attacked Mozart just because Karl Barth admired Mozart and wrote a book on Wolfgang Amadeus Mozart (guilt by association). And so, the following statement by Barth in the preface of CD III/4 in 1951 is understandable:
In the preface to CD III/4, Karl Barth wrote "This gives me occasion to say something rather sharp in passing. That the Neo-Calvinists in the Netherlands and elsewhere are not among my well-wishers is something that I have been forced to recognise at all stages of my path so far. Let us not blame them for this, nor for accusing me of being a “monist,” which they have recently proceeded to do. But it is going too far that in their attacks, obviously to offend me the more, they so far forget themselves as to use unrepeatable terms in disparagement of W. A. Mozart. In so doing they have, of course, shown themselves to be men of stupid, cold and stony hearts to whom we need not listen." 
I highly admire the Dutch Calvinist theologian Gerrit Cornelis Berkouwer. He wrote an impressive book on Karl Barth called The Triumph of Grace in the Theology of Karl Barth (1954) that appreciated Barth's theology from within the Dutch Calvinist tradition. I loved this book when I read it because G. C. Berkouwer critically appreciated Barth's work without resorting to demonic snickering like Berkouwer's contemporaries. Uncle Karl read this book and was also deeply moved by it (as I was), and praised it in the preface to CD IV/2 in 1955 where Barth apologizes for his global condemnations of the Dutch Reformed Calvinists (i.e. The Fundamentalist Calvinists). It was a very precarious and brave move by Berkouwer to write a book about Barth while Barth was in his theological prime, because he opened himself to correction by Karl Barth and condemnation by his Fundamentalist Calvinist contemporaries. I believe that Berkouwer's book is a testimony and witness to Calvinists everywhere that Karl Barth is their friend and not their enemy. Barth's willingness to apologize and recant in the following preface of CD IV/2 demonstrates that Calvinists everywhere have many things to apologize to Barth.
As I hurry to the end of this Preface [of CD IV/2], I must not forget to make some necessary amends. I am not referring to the strange (but not entirely novel) confusion which caused me (somewhere in CD IV/1) to transport the land of Israel to the western shores of the Mediterranean. I am thinking rather of the fierce attack which I made on Dutch Neo-Calvinists in globo [each and every one] in the Preface to CD III/4. The wrath of man seldom does that which is right in the sight of God, and never when it is in globo. I have to acknowledge this now that I have come to know the great book on myself and the Church Dogmatics by a representative of that group, G. C. Berkouwer (The Triumph of Grace in the Theology of Karl Barth, 1954). For all its reservations and criticisms this work is written with such care and goodwill and Christian aequitas [fairness] that—in the hope that there are others like its author—I should like to withdraw entirely the generalized and therefore ill-founded words which after many years of provocation I then suddenly unleashed. There are obviously “Fundamentalists” with whom one can discuss. Only butchers and cannibals are beyond the pale (e.g., the one who summarily described my theology as the worst heresy of any age), and even they only provisionally, for there is always hope that they will attain to a better mind and attitude. Those who were wounded then can take comfort in the fact that I myself have now come under the charge of “Fundamentalism,” and indeed of an “existentialist Fundamentalism” (whatever that may be). And if in the future they do not say any more unseemly things about Mozart, they need have nothing to fear from me. 
Berkouwer was right to praise Barth's theology as a triumph of grace, and he was one of the first Calvinists to recognize the achievement of Barth's work and boldly speak out in Barth's defense while the rest of the Calvinists were saying every vile thing they could imagine to attack Barth. Later, many other Calvinists and American Evangelicals followed Berkouwer's example and began to appreciate Barth for the first time. Barth's apology was also helpful, for course correcting the Dutch Calvinists, and demonstrating that they are both Calvinists and may benefit each other. Barth was not apologizing to the Fundamentalist Calvinists, but to those Calvinists who appreciated his work, and Barth's influence among the Dutch Calvinists was healed for many (but the Fundamentalist Calvinist sickness remains to this day). It wasn't until much later, after Barth had visited America in the 1960's and his Church Dogmatics were translated into english, that the American Calvinists began to repent of their Fundamentalist Calvinist sickness as well and began to appreciate Barth and Berkouwer.
If you haven't read Berkouwer's The Triumph of Grace in the Theology of Karl Barth, then I highly recommend buying it and reading it today!
1. Karl Barth. Church Dogmatics, The Doctrine of Creation Vol III/4. trans. G. W. Bromiley, T. F. Torrance, Edinburgh: T&T Clark 2004. Preface.
2. Karl Barth. Church Dogmatics, The Doctrine of Reconciliation Vol IV/2. trans. G. W. Bromiley, T. F. Torrance, London; New York: T&T Clark 2004. Preface.
3. Header: Portrait of G.C. Berkouwer (source wikipedia).
Related: Church Dogmatics, Dutch Calvinists, Dutch Reformed, Fundamentalism, Fundamentalist Calvinist, Fundamentalists, G.C. Berkouwer, Gerrit Cornelis Berkouwer, Karl Barth, Triumph of Grace in the Theology of Karl Barth