“His smile is like an infant's”- Whose smile is like an infant's? Why is his smile compared to that of an infant? Why does the poet say like that?

The smile of the young soldier is like an infant's.
   
               The soldier is sleeping peacefully in the valley. He has a smile on his face, which is gentle and without guile. He has died young having not experienced any complexity of life. Therefore, his smile is innocent like that of a child.

           Here the poet expresses how terrible the effect of war can be. The young soldier was innocent, and he had no experience of warfare at all. He was forced to depart from this beautiful world when his life was beginning to grow. The poet hates war. So, the poet asserts this line.