Rupert Brooke is often criticized for not being realistic about war. To put it another way, his poetry—and the war sonnets in particular, of which "The Soldier" is one of the most famous—is idealistic.
In "The Soldier," for example, the speaker says nothing about the horrors of war. The mass murder of millions of young soldiers over inconsequential plots of land, for example, is nowhere mentioned or even hinted at in Brooke's poem. Instead, the speaker suggests that dying on the battlefield while claiming more land for one's country is a noble, a heroic, even an ideal way to go out.
Moreover, the poem says nothing about the gaping void the soldier's death will leave in the lives of his friends and family. All we get is a description of a peaceful death that leads the soldier to an even more blissful "English heaven." The realities of war—death, sadness, loss, you name it—are not to be found in "The Soldier," or Brooke's work in general. You'll have to look elsewhere for that stuff.