Julius Caesar and his Invasion of Britain
Invaded Britain: Twice! In 55 & 54 BC (before he became the ruler of Rome)
What happened? He and his army of 25,000 men did a lot of fighting and won a lot of battles. But they didn't stay in Britain.
Why did he leave? Julius Caesar and his army weren't prepared for the weather and there was trouble brewing in other parts of the Empire. Also the local tribes had begun to rebel against him.
Did he come back? Caesar always wanted to conquer Britain properly, but never got the chance to return before he was assassinated by a group of conspirators on the Ides of March (15th of March, 44 BC)