What does the poem "In Flanders Field" teach you about the nature of war?

I would say that this poem is trying to tell us that war
is sad but it is also in some way honorable or at least
necessary.

We can clearly see that war is tragic in the
first two stanzas.  The speaker talks about how he and the rest of the dead were
vibrantly alive just a few days ago.  They could enjoy nature, they could love and be
loved, but now they are dead because of the war.

But that
is not all there is to war.  The last stanza shows that the war is important and that
they are not upset to have died.  Instead, they seem to feel proud of their actions and
they expect the living to continue the fight.