Per Lord Alvanley in Touteng v Hubbard (1802) 3 P&B 291 at p.301:
The principle established by these cases [Paradine v Jane (1647) Aleyn, 26; Blight v Page (1801) 3 B. & P. 295] appears to be this; that if a party contract to do any thing, shall be bound to the performance of his contract, if from the nature of that contract it is capable of being performed, and legally may be performed. But where the policy of the state intervenes and prevents the performance of the contract, the party will be excused; and so if a party who has contracted no to do something is directed by act of Parliament to do that very thing, he is released from his covenant.
Geipel v Smith (1872) 7 QB 404, per Blackburn J at p.413:
Here there was an executory contract, and the object of both parties had depended very greatly on time. The goodsowner stipulates to get his coals delivered with reasonable time, and it would be monstrous to say that, in the event of a blockade, he was bound to provide a cargo and keep it on board all the time, until, at the very least, all commercial profit would be at an end. On the other hand, it would be an equal hardship on the shipowner were he bound to keep his ship in the dock until it perhaps rotted. The object of each of them was carrying out of a commercial speculation within a reasonable time; and if restraint of princes intervened and lasted so long as to make this impossible, each had a right to say, "Our contract cannot be carried out;" and, therefore, the shipowner had a right to sail away, and the charterer to sell his cargo or refrain from procuring one, and treat the contract as at an end.