。
The soldiers were marching with their swords gleaming.
Literal
Soldiers [topic-は] swords [object-を] sparkling made-[while-ながら] were-marching.
きらきら is a 擬態語 (state mimetic) for bright, sparkling light. させながら combines the causative ~させる ('make it sparkle') with ~ながら ('while'), painting a vivid image: swords made to glint as they march. The causative here is natural — the soldiers cause the swords to gleam by brandishing them.