A moonscape of craters, mud and shattered stumps fills a wall-sized video screen; you can hear shrieking shells and shattering blasts; an enormous British howitzer, meant to pulverize the enemy’s defenses, points toward the fields. The only thing missing in this gallery, devoted to the Battle of the Somme at the Imperial War Museumhere, is the ability to conceive of 20,000 British dead and 37,000 wounded or missing in the first day of fighting, and more than a million casualties over all during five months.

It is one of the most powerful presentations at the new First World War Galleries here, suggesting that this seemingly futile battle was actually a turning point. These galleries, which replace an older presentation that was a classic for a generation, are also part of a $67 million rebuilding of the museum, completed in time to commemorate the centennial of Britain’s entry into the war. That occasion was somberly observed across Britain on Aug. 4 with moments of silence, extinguished lights and the scattered petals of red poppies — the war’s symbol of bloodied innocence and death.

At the museum, the artifacts and images are much less delicate. A stream of stretchers bearing the wounded winds through a silent 1916 film about the Battle of the Somme. Another display shows “geophones”: sonic sensors used with stethoscope ear pieces to detect the enemy burrowing tunnels. We see a tube-shaped soft helmet infused with caustic chemicals meant to counter phosgene gas — a poison almost undetectable, we learn, except for its scent of moldy hay. One case is filled with an array of nail-studded clubs used in combat and evoking medieval dungeons.