Great Britain is an island and the only way that there came a time when the “sun did not set upon the British empire” is the fact that in its heyday, Great Britain had the most formidable navy in the world, bar none. While it is probably true that conscripted soldiers aren’t treated all that well in the best of times, it therefore pretty much follows that conscripted sailors stationed in the vast wide ocean on a ship, of which unquestioned discipline is part and parcel of how navy ships are able to get things done, are going to find that insubordination is not going to work out well for those so inclined to speak up about their rights, real or imagined. This pretty much signifies that service in the Navy in the late 18th and early 19th centuries was the type of job that most people of sound mind would not willingly desire to take, and therefore the British Royal Navy found itself many a time, with a serious labor shortage of sailors.
One way utilized so as to get a given quota of sailors for a naval ship was to basically kidnap or coerce the drunk and disreputable from taverns and things of that sort; though, of course, the quality of the man so snatched from there was typically rather suspect. A far better way, though, to add sailors was to reclaim those that were said to have deserted the service, of which in a day and age, in which identification of who and what a given person represented -- being somewhat hard to verify, one way or another, this so provided the reasoning to question sailors on other ships, and therefore when, for instance, merchant ships, were boarded by the British Royal Navy, the Royal Navy took it upon themselves to take those that they claimed to have deserted them, and thereby “impressed” them back into service with the British Navy.
All of the above sounds rather eerily similar to the fugitive slave law, in which those that were alleged fugitives of the law, and considered by those in the business of capturing such, to be by their rights to catch them, of which, upon so being caught, then that person would be returned or “impressed” back into slavery. So then, in that day and age in which Britain ruled the seas, it was Britain, that acted as judge, jury, and executioner, and if Britain decided that certain sailors of a United States merchant ship or even of a frigate, were in their viewpoint, really British subjects -- that had wrongfully abandoned their duty to Britain, then Britain would claim them as their own, and impress them back into service with Great Britain.
Not too surprisingly, sovereign nations don’t take too kindly to their citizens being manhandled and kidnapped by any foreign nation, especially one that they had to previously fight so as to gain their own liberated freedom. Yet, at a time, in which the United States Navy and military power were really not a match to Great Britain, America thereby swallowed its pride and permitted that which they did not desire to see permitted, to be done. Still, there did come that time, as in the War of 1812, in which part of the reason for that war, was that America was sick and tired of being treated as a cat’s paw by Great Britain, and therefore they went to battle, to demonstrate that America was the wrong nation to be tread upon, so as to do right by those that were personally suffering from having been so wronged.