In the first Thor movie, after Thor's shenanigans on Jotunheim, Laufey presents Odin with a declaration of war, and then Odin takes everyone back home to Asgard through the Bifrost.

...and then what? Aside from Loki's secret passage, which wouldn't have remained a secret very long (and probably could have been sealed off very quickly) if someone tried to march an army through it, the only way we know of to get from Jotunheim to Asgard is the Bifrost, which is controlled from Asgard's end. Odin seems to take the threat very seriously, seriously enough to cast Thor down to earth directly in the path of Jane's van, but what sort of threat was it really? Can you really be worried about a war where your enemy can't reach your land to attack you?

In the first Thor movie, after Thor's shenanigans on Jotunheim, Laufey presents Odin with a declaration of war, and then Odin takes everyone back home to Asgard through the Bifrost.

...and then what?

Aside from Loki's secret passage, which wouldn't have remained a secret very long (and probably could have been sealed off very quickly) if someone tried to march an army through it, the only way we know of to get from Jotunheim to Asgard is the Bifrost, which is controlled from Asgard's end.

Odin seems to take the threat very seriously, seriously enough to cast Thor down to earth directly in the path of Jane's van, but what sort of threat was it really?

Can you really be worried about a war where your enemy can't reach your land to attack you?