Dilip Hiro traces the bitter contours of that rivalry in “The Longest August,” a history that spans a century of antagonism, skulduggery and war, from the tensions between the Hindu and Muslim leaders of the British Raj, in the early 20th century, up to the horrors of the 2008 attacks on Mumbai and the most recent, failed efforts to make peace between these estranged siblings. It is a tale of broken bloodlines, fatal miscalculations and mutual paranoia that has placed a bitter parting at the center of the destiny of a subcontinent. And it is timely: As American troops beat a hasty retreat from Afghanistan after 14 years of troubled intervention, the region’s most intractable dispute is now surging back to the surface.

Peace between Pakistan and India has long been seen as the region’s diplomatic holy grail, the key to unlocking hostilities from Kabul to Kashmir. Yet, as “The Longest August” comprehensively demonstrates, history offers few reasons to hope it might happen any time soon. Hiro, a veteran freelance journalist based in London, dutifully covers much ground. He skillfully teases out the contradictions of leaders on both sides, and is generally fair-minded in apportioning blame for their many failings.

Much of the story, though, is dominated by the boom of artillery as Pakistan and India flirt with war, and occasionally indulge in it. Hiro takes an unswerving view of the frequently negative role played by foreign powers like China and the United States, which sought to leverage the rivalry between Pakistan and India to their own ends. He spotlights the Nixon administration’s shameful role during the 1971 war in East Pakistan (now Bangladesh), when American officials turned a blind eye to Pakistani war crimes — territory that was comprehensively covered in Gary J. Bass’s “The Blood Telegram.”

Hiro also offers fascinating accounts of the espionage wars between the two countries, some of them half-forgotten today. He reminds us that, during the 1980s, Gen. Mohammad Zia ul-Haq of Pakistan secretly armed Sikh separatists in India, while Indian leaders plotted with Israel to bomb Pakistan’s clandestine nuclear facilities — an attack that never came to pass.

Inevitably, perhaps, the story is ­weighted toward turbulent Pakistan, which has produced more quotable villains. “Drinking, gambling, dancing and music were the way the officers spent their free time,” says the Islamist General Zia. “I said prayers, instead.” Yet the political drama is interrupted by anecdotes about the movie industry, and how Bollywood musicals have succeeded where politicians have failed in bringing Indians and Pakistanis together.