In the immediate aftermath of World War I, class conflict increased, much to the dismay of the middle class Progressive reformers. Coolidge (who was the governor of Massachusetts in 1919) was among the vanguard of politicians that realized that the nation was ready to move on from the seemingly non-stop class conflict (e.g. labor strikes) that had regularly occurred since the end of the Civil War.
By 1919, it was widely known and accepted by most everyone in Boston that its police
force was underpaid and overworked. Labor strife became more widespread after World War I, in part due to the vast shortage of jobs with about 1 million veterans returning home from Europe (there was no equivalent of a G.I. Bill). During the war, laborers in city-after-city were promised that once the war ended, wages would increase. However, city and state governments were unable to do so after the war, since the "velocity of money" (consumer spending) had stalled, leading to a short but brutal post-war recession. In this post-war landscape in Boston, the Industrial Workers of the World (IWW, nicknamed the "Wobblies", an aggressive Socialist worker's union) was able to establish a level of influence with a significant number of Boston police. Their gamble: by walking off the job for higher pay and shorter hours, other laborers in Boston would also go on strike in support (a General Strike, shutting down all services in Boston). In particular, the striking Boston police were hoping that the ladies that operated the city's telephone switchboards would join them, igniting the General Strike: Boston and Massachusetts politicians would then undoubtedly be forced to negotiate.
The striking workers had reason to believe their strategy would be effective, in that Governor Calvin Coolidge had a history of taking a "Middle of the Road" approach to labor conflicts in the recent past. In September of 1919, the vast majority of Boston police went on strike - the city's social order collapsed almost immediately. Similar to the chaos in Los Angeles in 1991, a percentage of Boston's population decided to take full advantage of the absence of law enforcement. Reports of assaults, theft, and even deaths became commonplace, and requests for gun permits skyrocketed. The Boston Police Strike became national news, in that an economic and political precedent may be set - if the Boston Police Strike was effective, then other city's police forces would also go on strike (the Washington, D.C. police force was, in essence, at the starting line, ready to strike at a moment's notice).
President Woodrow Wilson was focused on his national speaking tour promoting the League of Nations, trying to put pressure on the U.S. Senate to reconsider its vote rejecting U.S. involvement in the League. Added to that were his efforts at trying to avert a national steel strike, and, to cap it off, his health was failing fast - he would suffer a very debilitating stroke during his national tour. Therefore, it was predictable that the President didn't place enough importance on the Boston Police Strike, although while in Montana, he would eventually endorse Coolidge's actions in ending the strike.
Governor Coolidge was in a tough spot - it seemed the only real choice was to negotiate with the police labor leaders, giving them at least most of what they wanted so the city would be safe once again. Coolidge's decision stunned most observers - he in essence fired all the striking police men, vowing to never hire them back; he then ordered the entire state's militia (National Guard) into Boston to quell the spreading violence and lawlessness, citing the state constitution's clause that allowed the governor to act as "Commander-in-Chief" in emergency situations. Coolidge also was lucky - the ladies that operated the telephone switchboards decided not to strike, ensuring that communication was still in place to use the militia as a temporary substitute police force.
Some historians argue that Coolidge either waited too long, or went too far with his decision, but it seemed to me that Schlaes argued that Coolidge didn't decisively act until he was convinced that a "Middle of the Road" solution would be not only disastrous for Boston, and Massachusetts, but also for the nation. Coolidge, with plenty of support and some precedence, stated that due to public safety, the police of the city of Boston had no right to strike whatsoever. Coolidge, showing that he did indeed have sympathy for the striking police, tried to help as many as possible find a job - but he did not allow them to come back to the Boston police force - he made sure that the striking police men were replaced by newly hired candidates.
Coolidge's political popularity in Boston and Massachusetts increased to even greater heights with his decision to use the militia. With the terms of elected office so short in Massachusetts in those years, Coolidge was up for re-election for governor very soon, but it turned out that he had become such a prominent Republican politician as a result of ending the strike, that he was on the short-list of Republicans for the Vice-Presidential slot in 1920. Calvin Coolidge, not President Wilson, was the politician that was out in front in dealing with the Boston Police Strike of 1919; Wilson was mostly focused on the past, with his League of Nations tour, while Coolidge realized that America was at the brink of a new era. Schlaes argued that 1919 was similar to 1787; America was ready to move on from a major war, and Calvin Coolidge was among the first major politicians after World War I to recognize and act on that belief.
With the election of the Republican Warren Harding as president in 1920 (w/ Coolidge as his Vice-President), the Progressive Era was officially at an end. Harding received over 60% of the popular vote, campaigning on the slogan "A Return to Normalcy". In 1923, Harding died of natural causes, and Calvin Coolidge became the 30th President of the United States, winning election in his own right in 1924.
New York Times article on Amity Schlaes' Coolidge (14 February, 2013)