Final answer:
The Tet Offensive was a major political victory for the Vietcong, significantly altering U.S. public opinion and leading to President Johnson's decision to seek an end to the Vietnam War.
Step-by-step explanation:
The Tet Offensive marked a significant turning point in the Vietnam War because it was a major political victory for the Vietcong (VC). Although resulting in substantial casualties for the VC, the magnitude of the attack during the Tet, the lunar New Year, surprised the US military and revealed that the previous US claims about the war's progression were overly optimistic. The brutal images of the fighting broadcasted on television and the unexpected scale of the offensive shifted American public opinion significantly. Many Americans began to question the presence of US troops in Vietnam, and this led to President Johnson looking for ways to end the war.