What man seduces Helen resulting in the Greeks going to war with the Trojans?
Let me give a little background, which may help a little. Paris is the person who seduces Helen from Menelaus. And it is on account of this action that the war starts between the Greeks and Trojans. The back story is slightly more complicated. There is a divine wedding banquet between Thetis and Peleus, but Eris is not invited. So, Eris seeks revenge by sowing discord and envy. So Eris throws an apple, which was to go to the most beautiful goddess, but there were three goddess present: Hera, Athena and Aphrodite. And they all wanted it! Zeus did not want to make a decision of who was most beautiful. So, he stated that Paris would make the decision. At this point the goddess offered various things to sway Paris. Hera offered to make him a king; Athena offered wisdom and skill in battle; Aphrodite offered the love of the most beautiful woman. Paris choose Aphrodite's offer. So, who really stated the war? Eris? Zeus? Paris? It is complicated.
It is also important to understand that Helen, a Greek, is Menelaus' wife. According to mythology, Paris was actually awarded Helen because she was the most beautiful woman in the world. Aphrodite, the goddess of love, awarded Paris Helen. Unfortunately at the time, she was indeed married.
She is often referred to as Helen of Troy, but that is a misconception. She was Greek. Paris went to get his prize and after being welcomed by both Menelaus and Helen, stole Helen when Menelaus was away from home. Paris took her from Greece to Troy with him. Menelaus alerted his partner Greek kings and their armies joined together to get her.
The name of the man that you are thinking of is Paris.
Paris is a very prominent man from the city of Troy. In fact, his father is Priam, who is the king of Troy at the time that Paris abducts Helen.
As you say, when Paris seduces and abducts Helen, the war between the Greeks and the Trojans breaks out. In this war, Paris plays an important role.
Even though he is really not much of a warrior (his brother Hector is the real warrior) he is the one who kills Achilles by shooting him in the heel with an arrow.