At the head of a picked corps, the young king rode south from Pella, taking the coast road through Methone and Pydna into Thessaly. When he reached the Vale of Tempe, between Olympus and Ossa, he found the pass strongly defended. The Thessalians told him to halt his army while they made up their minds whether or not they should admit him.
Alexander, with dangerous politeness, agreed – and at once set his field-engineers cutting steps up the steep seaward side of Mt Ossa. (Traces of these steps, known as ‘Alexander’s ladder’, still survive.)
Before the Thessalians realized what was happening, he had crossed the mountains and was down in the plains behind them. With their flank thus neatly turned, they chose to negotiate rather than fight. Alexander – having made his point – was all charm and friendliness.
Source:
Green, Peter. “The Keys of the Kingdom.” Alexander of Macedon: 356-323 B.C.: A Historical Biography. Univ. of California Press, 2005. 116-17. Print.
Further Reading:
Alexander III of Macedon / Ἀλέξανδρος ὁ Μέγας (Alexander the Great)
the story has many sources on the web
but I am disappointed that photos are not particularly quick and easy to find