What was Big Willy Shakes going for?
Henry is literally telling his men to rush into the battlefield. He says "once more to the breach" but what he means is, "We've just blown a giant hole in the town's wall so please rush in there ASAP, even though it's dangerous and you'll probably die." It doesn't sound so rousing to us.
But it works. Henry knows when—and how—to cheer his men on. And it turns out his speech is really telling them so much more than that. He calls them "dear friends" as if they're all in this together. That sense of patriotism and morale is what makes Henry a great leader in the play.
Never mind that he ordered the invasion and risked all of their lives in the first place. Forget about the fact that he will benefit from the rewards of gaining yet another crown (while life will probably go on as normal for the rest of 'em). On the battlefield, in this speech, he's a soldier. And that's something the men need to see and hear as they run off into battle, or into the breach.
This little speech of his does the trick. His men become empowered and fight harder for him than we even thought they could. Eventually, Henry secures the French crown for himself, and the rest is history.