France, being a powerful country, is glad enough, when pressed by the English, to have diversions made for her, whether in Scotland or Ireland; but she has no idea of putting herself out, for the sake of her allies, when she desires peace with England." France had indeed been quick to take advantage of the trouble caused to Henry by the rising in the north.

While he was gathering his army, although there was a truce with England, a French expedition, in which many of the royal princes took part, had invaded Guienne, captured several castles held by the English adherents, made frequent descents on our coast, plundered every ship they met with, captured a whole fleet of merchantmen, taken the islands of Guernsey and Jersey and, while Henry was fighting at Shrewsbury, landed near Plymouth and plundered the whole country round.

On the news reaching them of the result of the battle of Shrewsbury, they at once burned Plymouth to the ground, and then, re-embarking, sailed for France.

All remonstrances on the part of Henry were met by declarations that these raids were carried on without the knowledge of the French king, and were greatly against his inclinations, which were wholly for the strictest observance of the truce.
Nevertheless, a few months later, the Count of Saint Pol landed a force in the Isle of Wight; but the people of the island rose in arms, and defeated the invaders, who sailed hastily away.
Although, having other matters in hand, Henry professed to believe the French king's assurances; the sailors and ship masters were in no way content to suffer unresistingly, and the men of the seaports of the east coast, and of Plymouth and Fowey, banded themselves together, and carried on war on their own account; capturing several fleets of ships, loaded with wine and other valuable commodities; burning the coast towns; and making several raids into the interior of France, and carrying off much plunder.
Enraged at this retaliation, the French incited the Flemings, Dutch, and Hollanders to cruise against the English; and these, sailing in great ships, executed so many atrocities upon English crews and ships that, later, Henry himself sent out a fleet, under his second son, who executed his commission, effectually destroying ships, burning towns, and putting the people to the sword without mercy.
Thus the breaches of the peace by the French recoiled terribly upon themselves, and they suffered vastly greater loss than they had inflicted upon the English.
From the time when he let slip the opportunities, both of joining Hotspur and of falling on the royal army after their victory, Glendower's power declined.

For a time he continued to capture castles, and to carry out raids across the border, but gradually he was driven back to his mountain strongholds.