It is considered a major sin to break a contract, if you agreed, especially with military situations.

Meaning of the quote

Breaking a promise or agreement, especially one related to the military, is seen as a very serious wrongdoing. If you make a commitment, you should keep your word and follow through, even if it's difficult. Going back on your agreement is considered a big mistake that can have serious consequences.

About John Walker Lindh

John Walker Lindh, an American who joined the Taliban in Afghanistan, was captured during the US invasion in 2001. He was detained, tried, and sentenced to 20 years in prison. Lindh, a convert to Islam, received training at al-Qaeda’s Al-Farouq camp and was involved with the Taliban’s fight against the Northern Alliance.

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