A brief reading of the sentences suggests that the paragraph is about the dispute between the US and Canada over the Northwest passage. 2 is a better opening sentence than 1 as 21 forms a good introduction into the dispute. 2 mentions that Canada claims ownership over the passage, and 1 then mentions the counterclaim the US offers: the passage is an international route. 43 becomes a mandatory pair which follows 21. 4 mentions the historical development in the dispute: it has not been legally settled yet. 3 then hints about the future of this dispute: the dispute itself would vanish once the entire Arctic begins to stay ice-free in summer. Hence, the coherent arrangement is 2143.