They had been the means of placing Henry on the throne, as it would seem, without intending it, for he had sworn to them on the Gospels at Doncaster that he aimed at nothing more than asserting his own invaded rights. Henry had rewarded them with large grants of land, including those of their prisoner Douglas, which lay in Eskdale, Liddesdale, with Ettrick Forest, and the lordship of Selkirk. The Percies, indeed, might regard these last as scarcely more than nominal gifts, for they would require a powerful force to keep possession of them, and they were almost immediately retaken by the Scots. The Percies, in fact, were ill-pleased with the haughty tone of Henry, who owed them so much, and they were now in close alliance with the Mortimers, who had the real claim to the throne. That Henry received their desire to liberate their royal relative with fear and suspicion was clear from the fact that he made no resistance to the ransom of Lord Grey de Ruthin. Henry did not hesitate to say in reply to Hotspur's pertinacious demands of March's liberty, that he and his uncle Mortimer had gone to Glendower of their own accord, and that no loyal subject would, therefore, wish them back again.
This was pointed language to a mind like Hotspur's. But there were still other causes at work. The Earl of Northumberland attended at Westminster with his prisoner, Murdoch Stewart, the son of Albany, and six other captives. They were presented to Henry, who, though he invited them to dine with him, received them rather coldly, and used severe language to Sir Adam Forster, one of them. The earl pressed Henry for the payment of large sums of money due to him for the custody of the Marches and the costs of the Scottish war. This of all subjects was the most distasteful to Henry, who was always short of money, and reluctant to part with it when he had it. To balance this account, he now gave Northumberland, instead of hard cash, the lands of Douglas, which would require for their defence still more hard blows. Northumberland returned home in no good humour, and the work of revolt now went rapidly on.
The Earl of Worcester, Northumberland's brother, entered into the quarrel regarding the Mortimers. Scrope, the Archbishop of York, the brother of William Scrope, Earl of Wiltshire, who had been put to death by Henry at Bristol, and who, therefore, hated Henry, advised these nobles to depose the usurper, and place the Earl of March, the rightful prince, on the throne. The first open evidence of the insurrection was furnished by Edmund Mortimer, who, to free himself from captivity, gave his daughter in marriage to Owen Glendower, and on his part agreed to join the confederacy for the overthrow of Henry of Lancaster, with 12,000 men.
Meantime, the Percies and the Earl of March had agreed to liberate Douglas, their prisoner, on condition that he should join the enterprise with a certain number of Scottish knights. Accordingly the Percies and March made a foray into Teviotdale, and challenged the chivalry of Scotland, by way of concealing their real enterprise from the eyes of the English king, to meet them in battle on the 1st of August. Keeping up the appearance of an attack on Scotland, they invested an insignificant fortress on the borders, called the Tower of Cocklaws, commanded by a simple esquire, one John Greenlaw. This petty border hold was besieged with all the forms of war by this powerful army. It was assaulted by the archers, and battered by the trebuchets and mangonels, but still it stood firm, and its commander at length entered into a treaty with Hotspur, promising to surrender it in six weeks, that is, on the aforesaid 1st of August, if not sooner relieved by the King of Scotland, or Albany, the governor. This made it necessary to send a courier to Edinburgh, ostensibly to communicate this agreement to the Government, but really under cover of it to open a negotiation with Albany for his adhesion to the enterprise. The utmost publicity was given by the Percies to the expected rencontre between the nations on the 1st of August. They applied in all directions for aid and troops from their friends, and carried the deception so far as to even solicit Henry for arrears of money due to them, amounting to £20,000, in order to enable them to maintain the honour of the nation.
Henry must have lost much of his usual sagacity if he had not for some time seen through this solemn farce. The black clouds of the coming tempest had been drawing together from various quarters for some time, and dull must have been the vision of the Government had they not attracted their notice. Henry sent no money, but ominously avowed his intention of joining his faithful Percies in person, and sharing their dangers for their common country. This appears to have startled the covert insurgents. They at once altered the tone of their pretensions. They abruptly abandoned the anticipated glories of their Scottish campaign, and directing their course towards Wales, gave out that they were about to make war on Owen Glendower, in defence of King Henry.
Henry of Lancaster was by no means deceived. He knew that Mortimer had allied himself to Glendower, and publicly proclaimed his intention to maintain the cause of his nephew, the Earl of March against Henry. Still more, the Scottish Earl of March, refusing to participate in the treasonable designs of the Percies, from his mortal hatred to Douglas, whom they had made an associate, hastened to Henry, and fully apprised him of the real situation of affairs. Henry, therefore, lost no time in marching northward; but this movement quickened that of Hotspur.
It has been said that if this conspiracy had been executed with as much prudence as it was planned, it would have cost Henry his crown; and the cause of failure has been laid on the precipitancy of Hotspur and the timidity of his father. But it must be borne in mind that Henry was a suspicious and vigilant monarch, constantly in danger, and, therefore, constantly on the alert to detect it. Fortune, Providence, or his singular circumspection, served him uniformly in all these conspiracies, and enabled him to defeat his adversaries. It must also be borne in mind that to arrange a sufficient military force to overturn the throne of a monarch like Henry, it required extended ramifications of conspiracy; and this involved the imminent danger of bringing into the field of operation some individuals hostile or traitorous to the enterprise. On this occasion the Percies had announced their object to the Governments of France and Scotland, and the defiances arriving from the Duke of Orleans and the Count of St. Pol seem to have originated from this cause. But if they did not awake suspicion in the breast of Henry, there was the Scottish Earl of March, as there had been the traitorous Earl of Rutland before, to prove a stumbling-block to the conspirators. It was almost impossible to avoid making him a confidant, and if made, he was pretty sure to damage them through his hatred to Douglas.
At the critical moment when Henry had clearly obtained intelligence of what was going forward, Albany, who was raising all Scotland, and proposing to bring down 50,000 men to join them, had not had time to complete his muster. The old Earl of Northumberland fell ill, or, as some historians will have it, grew afraid, and could not march. It was, therefore, no precipitancy, but an inexorable necessity, which compelled Hotspur to use all diligence to effect a junction with Owen Glendower, before overtaken by Henry. He was accompanied by Douglas and his Scottish knights; and by his uncle, the Earl of Worcester, the lieutenant of South Wales, with what forces he could get together. The men of Chester always devoted to King Richard, joined them on the march to support his cause, for they heard that he was still alive. The whole insurgent army amounted to 14,000 men, and even though disappointed of the contingents of the Scottish regent and the old Earl of Northumberland, if they could reach the army of Glendower they would present a most formidable force.
But in this Henry was too quick for them. He himself, knowing the valour of the troops and the leaders who came against him, was desirous to delay awhile an actual conflict with them; but the Scottish Earl of March, who seems to have been an admirable tactician, as he had seen the true mode of action at Homildon, saw it in this case, and urged vehemently on Henry the necessity of checking the Percies before they could form a junction with Glendower. Henry saw the wisdom of the advice; he had now reached Burton-upon-Trent, and turning west, he pushed forward by forced marches, and entered Shrewsbury at the same moment that the advanced guard of Percy and Douglas was seen in all haste endeavouring to gain that city.