If David O. Stewart’s latest book, “Impeached: The Trial of President Andrew Johnson and the Fight for Lincoln’s Legacy,— was 25 chapters instead of 26, it might resemble something of a high-resolution snapshot — notably intricate in its illustration of complex detail, yet tragically frozen in time. But it isn’t.

As any talented lawyer would, Stewart saves his hardest-hitting arguments — in the book’s last pages — for the final appeal to his readers. His story, which begins as a historical narrative, concludes as a case study on the American impeachment process, a constitutional provision that, in the case of President Andrew Johnson, saved the country from a second civil war.

Johnson was never supposed to serve as president. Lincoln had chosen the Tennessee native to be his running mate in the election of 1864 because he appealed to Southern Democrats, yet he had sided with the Union during the war. In those days, picking a vice presidential running mate with opposing views was much more common.

When Johnson took office after Lincoln’s assassination, he faced an inevitably painful road to Reconstruction and an overwhelming Congressional minority for his party. At a time that called for a great leader, Stewart writes, Johnson was nothing more than politically shrewd, and stubborn to boot. After Johnson resisted countless Congressional attempts to rebuild the South under Union governance and integrate the newly freed slaves into society, Thaddeus Stevens and his majority of Republicans attempted to impeach Johnson on a technicality — a violation of the Tenure of Office Act.

The impeachment was ultimately a failure: Johnson was impeached by Congress but wasn’t removed from office. Stewart puts forth convincing evidence throughout the book that suggests Johnson’s acquittal was bought with bribes and political favors. He also blames a shoddily executed prosecution. Whatever the reason for the acquittal, as Stewart points out in his conclusion, the fact that the process took place and ended peacefully demonstrates the constitutional importance of impeachment.

“What this story says to us now is that impeachment is a useful tool for giving us a safety valve when there is dissatisfaction with a president,— Stewart, a trial lawyer in Washington, D.C., said in an interview. “It gives people time for second and third thoughts.—

In taking this view, Stewart contradicts a laundry list of notable scholars, including John F. Kennedy, who say Johnson’s removal from office would have caused a constitutional crisis because the basis for impeachment in his case was entirely based on political opposition. Stewart doesn’t argue directly against Kennedy’s view but instead insists that people like him were dwelling on the wrong questions. After all, the trial ended Johnson’s presidential power trip and prevented military conflict.

“I think in the short term, [the verdict] didn’t matter a great deal. The impeachment process itself greatly wounded— Johnson, Stewart said. “But I think in the end, we could have had a lot of violence in this country. Impeachment channeled that into a peaceful process.—

But impeachment is not without its flaws. During impeachment trials, Senators become judges. They alone decide whether to remove a president from office. How, then, can impeachment not become political? The lawyers who tried Johnson struggled to exclude political contamination from a process that, by its very nature, oozes political bias from every pore.

This recognition adds a rather confusing but necessary nuance to Stewart’s overall argument. Impeachment “is an unwieldy tool,— Stewart writes in the final chapter. “But an essential one.—

The Johnson impeachment trial took place in an America that, thankfully, no longer exists. That doesn’t mean, according to Stewart, that we can’t use it to refine our understanding of the words written in our own Constitution. Stewart believes that as the presidency expands its reach, we might be called on to employ impeachment once again. If the Johnson trial is any indicator of what a future one might look like, let us hope that is not the case.