In Robert Frost's poem "A Soldier," what is the obstacle the poet alludes to?

"A Soldier" is a sonnet written by Robert Frost not long after World War I. In it he begins by writing about a single soldier, who he describes metaphorically as a fallen lance, left behind on the battlefield, rusting. Frost then goes on to describe war in general and the ways in which it has changed, so those of us not right in the middle of it become detached from it. Towards the end, he writes,

"But this we know, the obstacle that checked

And tripped the body, shot the spirit on

Further than target ever showed or shone." (Frost ll. 12-14)

The obstacle of which he writes is the physical death of the soldier. The soldier's  life may have seemed meaningless in this war where he is one of thousands, but his death takes him to Heaven or to some sort of an afterlife that is filled with meaning.