Why do western armies always tell their enemies when they will attack?

[Salnax]

It's because the military action itself is only a part of the picture. By setting the time and place of the attack in stone, it sends a message to all the enemy's grunts. "We will fuck you whenever and wherever we feel like it. Go ahead, take a running start. I like my prey to squirm a bit." Incidentally, this also gives the less-willing recruits an opportunity to escape. The fact that democracies almost always have the power to win helps, AND it makes them look like the good guys.

The difference between that and, say, Germany invading Poland is that Germany was not exponentially more powerful than Poland, and Poland had the capacity to really hurt Germany's armed forces. The current phenomenon only exists because of vastly different power levels.