Tone is the poet’s attitude toward a subject.
The poem “In Flanders Fields” is about World War I. There are several different ways that I could describe this tone. For example, it could be pleading. I suggest pleading because the speakers are the dead soldiers, and they say “Take up our quarrel with the foe:/To you from failing hands we throw/ The torch” and this means they are passing the responsibility on, and want the reader to pick it up and fight their fight for them or they “shall not sleep.” This means they will not rest in peace unless they are avenged, so you might also consider the tone vengeful. I do not see a lot of bitter words though, most of them are melancholy and reminiscent, two other tone words you could use for the same reasons.