The six months following the affair of the Trent constituted a period of comparative calm in the relations of Great Britain and America, but throughout that period there was steadily coming to the front a Northern belligerent effort increasingly effective, increasingly a cause for disturbance to British trade, and therefore more and more a matter for anxious governmental consideration. This was the blockade of Southern ports and coast line, which Lincoln had declared in intention in his proclamation of April 19, 1861.

As early as December, 1860, Lyons had raised the question of the relation of British ships and merchants to the secession port of Charleston, South Carolina, and had received from Judge Black an evasive reply[502]. In March, 1861, Russell had foreseen the possibility of a blockade, writing to Lyons that American precedent would at least require it to be an effective one, while Lyons made great efforts to convince Seward that any interference with British trade would be disastrous to the Northern cause in England. He even went so far as to hint at British intervention to preserve trade[503]. But on April 15, Lyons, while believing that no effective blockade was possible, thought that the attempt to institute one was less objectionable than legislation "closing the Southern Ports as Ports of Entry," in reality a mere paper blockade and one which would "justify Great Britain and France in recognizing the Southern Confederacy...." Thus he began to weaken in opposition to any interference[504]. His earlier expressions to Seward were but arguments, without committing his Government to a line of policy, and were intended to make Seward step cautiously.

Possibly Lyons thought he could frighten the North out of a blockade campaign. But when the Civil War actually began and Lincoln, on April 19, declared he had "deemed it advisable to set on foot a blockade," and that when a "competent force" had been posted "so as to prevent entrance and exit of vessels," warning would be given to any vessel attempting to enter or to leave a blockaded port, with endorsement on her register of such warning, followed by seizure if she again attempted to pass the blockade, Lyons felt that: "If it be carried on, with reasonable consideration for Foreign Flags, and in strict conformity with the Law of Nations, I suppose it must be recognized[505]." The Proclamation named the original seven seceding states, and on April 27 Virginia was added. The blockade was actually begun at certain Virginia ports on April 30, and by the end of May there were a few war-ships off all the more important Southern harbours[506]. This method of putting a blockade into effect by warning at the port rather than by a general notification communicated to European governments and setting a date, involved a hardship on British merchants since they were thereby made uncertain whether goods started for a Southern port would be permitted to enter. In practice vessels on their first departure from a blockaded harbour were warned and permitted to go out, but those seeking to enter were warned and turned back. In effect, while the blockade was being established, Lincoln's Proclamation had something of the nature for the timid British merchant, though not for the bold one, of a paper blockade. This was not clearly understood by Lyons, who thought neutrals must acquiesce, having "exhausted every possible means of opposition," but who consoled himself with the idea that "for some time yet" British trade could be carried on[507].

Lyons was in fact sceptical, as he told Seward in a long conversation on April 29 of the possibility of blockading a 3,000 mile coast line, but Seward assured him it would be done and effectively[508]. The British press was equally sceptical, and in any case believed that the war would be of short duration, so that there need be no anxiety over next year's supply of cotton[509]. In Parliament Russell took the stand that the blockade, if carried on in accordance with international law and made effective, required British recognition and respect. He also defended Lincoln's "notification at the port" method, stating that it might seem a hardship, but was perfectly legal[510]. Thus there was early and easy acquiescence in the American effort, but when, in June, there was revived a Northern plan to close Southern ports by legislative action, Britain was stirred to quick and vigorous opposition. Lyons learned that a Bill would be introduced in Congress giving the President authority, among other powers, to "proclaim" the ports closed, thus notifying foreign nations not to attempt to use them. He saw in it an unexpected application of the Northern theory that the South was not a belligerent and had no rights as such, and he regarded it as in effect a paper blockade[511].