There are multiple ways to address this question. One hinges on the actual incident that prompted the passage of the Tonkin Gulf Resolution—the supposed attack on US vessels in the Tonkin Gulf by North Vietnamese boats. There were two US Navy destroyers that reported attacks on two separate days in August. The first, the Maddox, was fired upon on August 2, 1964. The second, called the Turner Joy, also reported an attack, but in what has sometimes been cited as an example of the "fog of war," later expressed doubt that an attack had taken place. The second attack, it seems clear now, did not happen. Because the Tonkin Gulf Resolution was passed (at Lyndon Johnson's request) in response to these two attacks, it could certainly be argued that the resolution, which granted sweeping powers to Johnson to conduct a massive military buildup in Vietnam, was based on a false premise, and therefore, in a word, wrong.

A second way to look at this question is constitutional. The US Constitution places the power to declare war in the hands of Congress, and it does so for a reason. By ceding to Johnson enormous power to "defend" American forces in the region, Congress essentially gave the President a blank check to wage war. While many people in Congress assumed that the President would have to return to Congress if more forces were needed, the Resolution, it could be argued, was a violation of the spirit of separation of powers as laid out in the Constitution. So it could be said that the Tonkin Gulf Resolution was wrong on Constitutional grounds as well.

The Gulf of Tonkin Resolution was passed by Congress in 1964 to give President Lyndon Johnson the ability to conduct a war in Southeast Asia without making a formal declaration of war. After this resolution was passed, Johnson rapidly escalated American involvement in Vietnam. The resolution was in reaction to the Gulf of Tonkin incident, in which the U.S. destroyer the U.S.S. Maddox was attacked on August 2, 1964, by North Vietnamese torpedo boats while in the Gulf of Tonkin. The Maddox and another American destroyer also claimed to be have been attacked by the North Vietnamese on August 4. A later National Security report determined that while the Maddox had been involved in a skirmish on August 2 that there was no engagement between the Maddox and North Vietnamese boats on August 4. Defense Secretary Robert McNamara claimed that the Maddox had been on a routine mission, but a later Senate Foreign Relations Committee report found that the Maddox had been on a reconnaissance mission to collect information about the North Vietnamese. 

By the late 1960s, the Gulf of Tonkin Resolution was widely considered a mistake, as it had granted Johnson broad latitude to conduct a war that became deadly and disastrous, and there were calls for its repeal. It was eventually repealed under the Nixon administration in January of 1971. Later, the War Powers Resolution of 1973, passed over Nixon's veto, required the President to seek a formal declaration of war from Congress to send U.S. troops into a military conflict.