Born on the Fourth of July

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In the summer of 1956 in Massapequa, Long Island, New York, 10-year-old Ron Kovic plays war in the woods and attends a Fourth of July parade. After watching President John F. Kennedy's inaugural address,[3] it inspires him to enlist in the Marines. Later, Kovic attends an impassioned lecture about the Marine Corps. He decides to enlist and misses his prom because he is unable to secure a date with his love interest, Donna. He confronts her at the prom and has a dance with her on his last night before leaving.
Now a Marine sergeant and on patrol during his second Vietnam tour in October 1967, Kovic's unit kills a number of Vietnamese civilians in a village, believing them to be enemy combatants. During the retreat, Kovic accidentally kills one of the new arrivals to his platoon, a younger Marine private first class named Wilson. During a firefight in January 1968, Kovic is critically wounded until a fellow Marine rescues him. Paralyzed from the mid-chest down, he spends several months recovering at the Bronx Veterans Administration hospital where the hospital conditions are poor with rats roaming the floors and the doctors and nurses ignoring patients and abusing drugs and the equipment being too old to be useful. He desperately tries to walk again with the use of crutches and braces, despite repeated warnings from his doctors.
In 1969, Kovic returns home, permanently in a wheelchair with his leg intact. Though he tries to maintain his dignity as a Marine, Kovic gradually becomes disillusioned despite being paralyzed and resorts to alcoholism. In Kovic's absence, his younger brother Tommy (Josh Evans) has already become staunchly anti-war, remarking to Kovic what the war had done to him. During an Independence Day parade, Kovic shows signs of post-traumatic stress when firecrackers explode. When he is asked to give a speech, a baby in the crowd starts crying. After being unable to finish the speech and wheeled off stage, he reunites with his old high school friend Timmy Burns, who is also a wounded veteran, and the two spend Kovic's birthday sharing war stories. Later, Kovic goes to visit Donna at her college in Syracuse, New York, where the two reminisce and she asks him to attend a vigil for the victims of the Kent State shootings. He and Donna are separated after she and her fellow students are taken away by the police at her college for demonstrating a protest against the Vietnam War. Ron goes to a bar, almost getting into a fight with a fellow Marine and after a heated argument with his mother, Ron's father decides to send him to Mexico.
He arrives in ("The Village of the Sun") that seems to be a haven for paralyzed Vietnam veterans. He has his first sexual experience with a prostitute whom he believes he loves, until he sees her with another customer. Hooking up with another wheelchair-bound veteran Charlie, the two travel to what they believe will be a friendlier village. After annoying their taxicab driver, they end up stranded on the side of the road. After arguing with each other, they are picked up by a man with a truck and driven back to the "Village of the Sun." Going back to Long Island, Kovic travels to Georgia to visit Wilson's parents and family. He tells them the true story about their son's death and confesses his guilt.
Kovic joins Vietnam Veterans Against the War (VVAW) and travels to the 1972 Republican National Convention in Miami. He tells a reporter about his negative experiences in Vietnam and the VA hospital conditions, that the Vietnam War is wrong, and that the Vietnamese people are a proud people fighting against the US for their independence, fueling rage from the surrounding Nixon supporters. His interview is cut short when guards eject him and his fellow vets from the hall and attempt to turn them over to the police. They manage to break free from the police, regroup, and charge the hall again, though not successfully this time. Kovic speaks at the 1976 Democratic National Convention, shortly after the publication of his autobiography, Born on the Fourth of July.

Question: What does kovic display during the Independence day parade ?
Answer: During the parade, Kovic shows signs of post-traumatic stress