Why did arab nations place an oil embargo on the united states in 1973?

The Arab nations of OPEC placed the embargo on the United States as a response to US support of Israel in its 1973 war against a coalition of Arab states led by Egypt and Syria.

OPEC stands for the Organization of Petroleum Exporting Countries.  Within that, there was also the Organization of Arab Petroleum Exporting Countries (OAPEC), formed in 1968.  In 1973, OAPEC said they would cut oil production "until the Israeli forces are completely evacuated from all the Arab territories occupied in the June 1967 war."  (The 1973 war was being fought to regain control of territories lost to Israel in 1967.)  Egypt and Syria were both members of OAPEC, and they and other Arab nations were seeking leverage in the struggle with Israel and positioning for post-war settlements.

And this too use this if needed