Death Sentry & Thanos vs Asgard [including Odin & Thor]
Thanos decides to take up gardening and covets the World Tree.
Death Sentry is just, well an idiotic lunatic and so he agrees to help Thanos get it.
Odin is all steely eyed resolute defiance and unwilling to hand it over. So all of Asgard prepares for war. Oh and Thor's there, just for ... Well, he might be useful.
Both sides get no prep. This is an impulsive decision by Thanos and DS just tagged along 'cos he's got nothing better to be doing with his time.