Why does the poet describe Grendel's war as lonely? Contrast the monster's war against the war waged by the Danes and Beowulf (that is, why their war is bloody but not lonely.) Support your answers with concrete examples.
Grendel's war is lonely because he is the only one fighting it. Others defend themselves against the marrauder, but Grendel is the aggressor. The text says he "nursed a hard grudge," which means he is carrying some kind of bitterness in his heart against something/someone. Like most cases of bitterness, the one who holds the grudge is really the only one hurt by this wasted emotion. This kind of bitterness is a lonely emotion. Thus Grendel's lonely battle.
I'm going to go a bit more general on this question, although I love your interpretation. But consider the two battles - between the Danes and Beowulf, the battle is man vs. man. Although at odds, they can understand each other and their dueling purposes. However, in Grendel's war against man, he is alone. He is alienated and misunderstood because he has no community and he does not even share common traits with his enemies.
Grendel's war is lonely because he is the only one fighting it aside from his mother who Beowulf kills early on. I will try to support this with answers from the text Beowulf however, I must admit my response to this question is heavily influenced by having read John Gardner's "Grendel" which is a psychological/existential look at Grendel as the outsider.
In the text we discover that God is against Grendel (Chapter III line 54) and against all 'monsters' in general as being sons of Cain (Chapter II lines 59-61) The poet's including this is supposed to hit home with the reader because for many people the absence of God is the most desperate situation one can imagine. However, it appears that Hrothgar's people do not know God either (Chapter III lines 60-75) but they have at least eachother's company.
In keeping with the theme of God against Grendel, there is evidence that although the Danes do not know the real God (Chapter III), God in fact comes to their aid in Chapter XI. In line 5 we find out that "the Glory of Kings had given a hall-watch" Therefore in keeping of the theme of loneliness being perpetuated because of a lack of God, here in Chapter XI is evidence of God on the side of the Danes.
The war between the Danes and Beowulf is indeed bloody but not lonely because in the end they have God on their side.
This is of course only one aspect which could be explored but I am running out of characters!