Key observed the British bombardment of Fort McHenry in 1814 during the War of 1812. He was inspired upon seeing the American flag still flying over the fort at dawn and wrote the poem "Defence of Fort M'Henry"; it was published within a week with the suggested tune of the popular song "To Anacreon in Heaven".

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How do you pronounce "M'Henry"?
1. / məkˈhɛn ri, məˈkɛn- /
2. / mhɛn ri, mɛn- /

Both are correct. I'd use the first one, but the second is not incorrect, given the spelling.