India and Pakistan agreed to an immediate ceasefire on Saturday, unexpectedly halting the worst fighting in decades between the nuclear-armed neighbors, just when their tit-for-tat strikes appeared to be spiraling out of control.

The ceasefire had been expected to bring a swift end to weeks of escalating clashes triggered by the mass shooting of tourists last month that India blames on Pakistan.

The truce came after several days of the countries’ most expansive fighting in decades. One Indian official said the country would “deal strongly” with any violations.