Seward: Lincoln’s Indispensable Man. By Walter Stahr. Simon & Schuster; 703 pages; $32.50. Buy from Amazon.com, Amazon.co.uk

FEW have held power through events as monumental as those faced by William Henry Seward. From the 1830s, when he became a state senator and then governor of New York, to 1869, when he stepped down after eight tense years as secretary of state, Seward’s life was consumed by slavery and secession crises, in which he sought above all to save the Union.

Yet he is familiar to American schoolchildren largely for his role in buying Alaska, a venture that was at one time known as “Seward’s folly”. Walter Stahr’s new biography offers an overdue reminder of the much broader scope of his work. Seward was, Mr Stahr asserts, America’s second-greatest secretary of state, giving way only to John Quincy Adams, the force behind the Monroe Doctrine.

Seward’s problem is that he is condemned to be in the shadow of Abraham Lincoln. It might not have turned out so. At the 1860 Republican Party convention, Seward, then in his second term as a United States senator, had been favoured to win the party’s presidential nomination when the lesser-known Lincoln snatched it from him. Needing Seward’s Washington expertise, Lincoln tapped him for secretary of state. So powerful did Seward remain that he was targeted by the April 1865 cabal that killed Lincoln, surviving with some nasty knife wounds.

As secretary of state, Seward proved contentious. Critics charged that he was the “acting president”, though, in fact, Lincoln knew well enough how to handle his underlings. (Lincoln and his cabinet will be the subject of a new Steven Spielberg film this autumn.)

Seward’s triumph in foreign affairs was keeping European powers out of America’s war. Britain and France hungered for the rebels’ cotton, but Seward threatened to make war if those countries recognised the Confederacy, and ultimately they did not. A stiff test arose in 1861 when a Union frigate stopped a British ship and captured two Confederate envoys found aboard. Britain was hugely offended by this seizure, which it deemed illegal, and threatened war against the Union. Ultimately Seward and Lincoln, who had no desire for new hostilities, backed down and released the envoys. It was the Cuban missile crisis of the 19th century, Mr Stahr writes, with a touch of overstatement.

The drama of events during Seward’s eight years as secretary of state sweeps the tale along. Mr Stahr offers a clear-eyed overview of his time under Andrew Johnson, Lincoln’s troubled successor. Seward stood by Johnson despite Congress’s work to impeach the president for mismanagement. He also endorsed Johnson’s side-stepping of the demands of more progressive Republicans, who had sought to guarantee far more favourable treatment for freed slaves as the southern states re-entered the Union.

With the war over, Seward found the time to devote to expansionism. He not only arranged to buy Alaska for $7.2m in gold—one senator declared his support for the treaty on the condition “that the secretary of state be compelled to live there”—but he also began planning for the acquisition of Hawaii and the construction of the Panama canal, both of which later came to pass. He also wanted to buy British Columbia, which would have connected the rest of America to Alaska, but “British honour” kicked in, among other factors, writes Mr Stahr, and he failed.

Mr Stahr’s account is comprehensive and painstaking, aided by his subject’s trove of correspondence. But he leaves a craving for more insight into Seward’s character and motives. Seward lacked Lincoln’s gentleness of humour and touch, but he was nonetheless a fascinating figure. A not-quite-abolitionist married to an abolitionist, a man who passionately defended the defenceless as an attorney and yet could also display, as a contemporary noted late in his life, “overpowering egotism”. The question “What would it be like to have a beer with Mr Seward?” remains somewhat difficult to answer.