Say what you like about their invading Poland every few decades, but Germans are men of their word.
As with Pearl Harbor, 9/11, and basically any tragedy ever, there are conspiracy theories that say the American government allowed the attack to go ahead because they wanted an excuse to enter the war and start whooping German ass. But when it comes to the Lusitania, that idea is a little more plausible.
For one thing, the official story at the time was that two torpedoes launched by a German submarine sank the ship, but it's since been proven that only one torpedo was launched. The cause of the well-documented second explosion remains a mystery, which is like Viagra for the kinds of people who Google jet fuel temperatures on their lunch break.
"The shot might have come from that icy knoll."
In 1982, the British government was forced to admit that the Lusitania was secretly carrying a shitload of munitions back to Britain. The Germans at the time defended their attack on the ship as a military action, but it was long denied by the British in order to bolster war sentiment against those evil, civilian-targeting Hun assholes. Once salvage crews started diving into the wreck, the Brits had to come clean, lest the workers inadvertently explode inside the ostensibly weapons-free site. This was confirmed in 2008, when salvage divers recovered a mother lode of weapons which officially never existed.