Why was the united states military surprised by the tet offensive?

The Tet Offensive had an early attack, which caught people off guard.

Explanation:

So as we know, The Tet Offensive was a major military offensive launched by the army of North Vietnam against the United States and the South Vietnamese Army during the holiday of Tet during 1968.  The purpose of the offensive was to strike military and civilian command and control centers throughout South Vietnam and to spark a general uprising among the population that would then topple the Saigon government, thus ending the war in a single blow. So both North and South Vietnam announced on national radio broadcasts that there would be a two-day cease-fire during the holiday. This early attack did not, however, cause undue alarm or lead to widespread allied defensive measures. When the main Viet Cong operation began the next morning, the offensive was countrywide in scope and well coordinated, with more than 80,000 communist troops striking more than 100 towns and cities, including 36 of 44 provincial capitals, five of the six autonomous cities, 72 of 245 district towns, and the national capital.

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