John McCrae's poem "In Flanders Fields" is one of the most famous poems of the First World War, and its predominant theme is the nobility of sacrifice. Written after the Second Battle of Ypres in early 1915, it quickly became popularized in pro-war propaganda literature in both the United Kingdom and McCrae's own native Canada. The imagery of poppies came to symbolize the lives lost in war—a symbolism still in place today in the United Kingdom and many Commonwealth countries, where red poppies are worn for Remembrance Day (November 11th), commemorating the end of the First World War, and wreaths of red poppies are placed on war memorials throughout the country in honor of the fallen.

McCrae's poem has been co-opted in the service of patriotic feeling, and a whole tradition of remembrance has sprung up based on his words:

Take up our quarrel with the foe:

To you from failing hands we throw

The torch; be yours to hold it high.

If ye break faith with us who die

We shall not sleep, though poppies grow

In Flanders fields.

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.