Tensions over these issues extended back to the creation of the Socialist Party in 1901, and, as Kagan shows, plagued the party, as well as its associated trade unions, all along. They came to a head after 1919, when an insurgent left-wing declared its intention to take over the party and turn it into a replica of the Bolsheviks. Ugly factional struggles ensued. The result was a decimated Socialist Party in competition with an even weaker Communist Party. If socialism ever had a chance to succeed, it was now lost.
Although we can't fully evaluate Kagan's thesis because the footnotes—and virtually all indication of the primary sources she used—are missing in the PDF version that's floating about cyberspace, it's clear that the thesis made several notable scholarly contributions. She ably positioned her argument against the dominant explanations previous historians had offered. Most historians attributed the weakness of socialism in the United States to factors external to the party or movement itself: an ethnically divided working class, the two-party political system, the relative affluence that blunted a working-class consciousness. Kagan, by contrast, insisted on the importance of factors internal to the Socialist Party. In response to those scholars, such as the sociologist Daniel Bell, who argued that socialists were doomed to failure by their own inability to engage in the give-and-take of real life politics, Kagan stressed socialists' practical accomplishments: founding and managing unions that advanced racial integration, promoted labor education, and offered services to members. (The International Ladies Garment Workers' Union and the Amalgamated Clothing Workers of America operated mini-welfare states well before the advent of the New Deal.)
Kagan also took on historians such as Ira Kipnis (whose name she uncharacteristically misspelled), who contended that the party's expulsion of labor radicals in 1912 led to its destruction. Kagan's thesis persuasively argues that the party actually grew stronger after 1912 and reached its peak during World War I. Finally, against James Weinstein—arguably the most authoritative chronicler of socialism's demise—who stressed the Russian revolution as the key explanation, Kagan insisted on the primacy of deep internal rifts. If there's a major weakness in Kagan's thesis, it's here. Weinstein presented compelling evidence that previous debates between "left-wing" and "right-wing" Socialists played a minimal role in producing the 1919 split in which radicals, acting on proclamations from Moscow, attempted to purge moderate leaders in preparation for an all-out social revolution, only to be expelled from the party. A critical point made by Weinstein was that many of the chief actors in the split were recent Russian immigrants who had shallow roots in the Socialist Party. Kagan's thesis would have been strengthened by a more careful engagement with his findings.
On the whole, Kagan writes without evident bias, analyzing quite evenhandedly the rifts—which at times she suggests were doomed to be insurmountable—between the revolutionary and reformist camps in the Socialist Party as well as in the International Ladies Garment Workers' Union. If anything, Kagan seems to have more sympathy for the centrist "constructivist" leadership than do many historians who write about labor and radicalism. Her overall point, made without stridency, is that sectarianism, caused mainly by misguided revolutionary hopes, should ultimately bear the burden for the party's demise. Socialists, she seemed to say with some sadness and frustration, have often been their own worst enemies. In places, her tone even implies that she may consider this an ongoing characteristic of the American left. "Radicals have often succumbed to the devastating bane of sectarianism," she wrote; "it is easier, after all, to fight one's fellows than it is to battle an entrenched and powerful foe." In any case, there is no question that Kagan wrote not a propagandistic celebration of socialism's heyday but a judicious account of its self-destruction—with the hope that the left might learn from past mistakes.
Finally, quite apart from whatever political leanings it suggests, Kagan's thesis tells us something else significant about her. As a piece of research and writing, it displays remarkable intellectual maturity. It shows an uncommon ability to absorb and synthesize a large amount of information on what was, for an undergraduate in 1981, a fairly arcane topic. Kagan also exhibited an intellectual nimbleness in grasping and explicating subtle distinctions among different positions and factions within the complex labyrinth of radical thought—one where many historians have gotten hopelessly lost. She issued confident yet prudent judgments, and wrote in a taut and at times elegant prose that surpasses in quality that of more than a few professional scholars. The almost total lack of scholarly jargon is itself a feat.
These talents leave little doubt that Kagan would have become a superb historian if she had wanted to be one. Of course, even back in the 1980s, there was a tough academic job market in the humanities, and given the way things have worked out for her with her career choices, she probably made the right call.
Update, May 24, 10:17 a.m.: The historian Ronald Radosh has offered a similar assessment of Kagan's thesis—both its arguments and its high quality. As Radosh notes, it's heartening when people of different political perspectives agree on what is good scholarship.