In the poem "In Flanders Field" by John McCrae what is the dominant emotion?

I would say that the dominant emotion here is patriotism. 
This is not one of the WWI poems like those of Owen that talk about how terrible the war
was.  This is a poem that asserts that the war was important and that the dead demand
that the living continue the fight.

We can see this very
clearly in the last stanza.  The speaker tells the living that they must take up the
torch that is being thrown to them.  The dead will not lie easy unless the living keep
fighting against the foe who killed them.

This is why I
call this a patriotic poem -- its main point is that the living must continue fighting. 
This tells us that the war has an important point and that the living must continue
fighting for that cause.