What interrupts the march in "Dulce et Decorum Est"?

shells

gunfire

poison gas

a fistfight

The march in the poem "Dulce et Decorum" was interrupted by a poison gas.

Explanation:

The presence of the poison gas created fatigue in the march and the boys started to fumble and feel clumsy as soon as they inhaled the gas unknowingly. There was one of the person who was yelling and looking like a man who stood in the fire. Soon the march and the entire plan was ruined and the unexpected circumstances took place.