In 1953 help destabilize their country and install puppet leader
In 1988 shoot down one of their airliners
In 1988 join in the Iran-Iraq war on the side of Iraq, attacking 2 Oil rigs and 2 warships
In 1995 Create a trade embargo on Iran
In 2002 Refer to Iran as Axis of Evil
In 2003 Turn down peace/trade offer from Iran
In 2003 Begin incursions and attacks on Iranian sovereign territory using Drones
In 2006 Use the CIA to fund £400m to known terrorist organisations against Iran
In 2007 Raid an Iranian Consulate and kidnap 5 people
In 2008 Increase this use of known terrorists against Iran

2011/2012

Speculate based on zero historical evidence,
move your forces to someone's borders
create a major provocation
assassinate a few scientists
send them a computer virus
sabre-rattle and insult the leaders
apply economic sanctions
repeatedly discuss attacking the country with their enemies
Let slip to journalists that your ally will attack them before Spring
talk about "time running out"
ignore the fact the other two major superpowers in the world don't agree with you
assume this country is stupid enough to use nuclear weapons like you did... etc etc etc etc...

This is how to ensure a war starts...