At least this time round the invasion would spread democracy, but Iran's oil has proved problematic in the past too:
"In 1953, Iran's prime minister Mohammed Mossadeq, who had been elected to parliament in 1923 and again in 1944, and who had been prime minister since 1951, was removed from power in a complex plot orchestrated by British and US intelligence agencies ("Operation Ajax"). Many scholars suspect that this ouster was
motivated by British-US opposition to Mossadeq's attempt to nationalize Iran's oil. Following Mossadeq's fall, Shah Mohammad Reza Pahlavi (Iran's monarch) grew increasingly dictatorial. With strong support from the USA and the UK, the Shah further modernised Iranian industry but crushed civil liberties. His autocratic rule, including systematic torture and other human rights violations, led to the Iranian revolution and overthrow of his regime in 1979. After over a year of struggle between a variety of different political groups, an Islamic republic was established under the Ayatollah Khomeini."
http://en.wikipedia.org/wiki/Iran