Lately I've heard a lot of people talking about how Trump is trying to start a war with Iran because he's looking to distract people from the impeachment hearings, or because he thinks it'll help him get re-elected. And while I don't have any doubts about Trump's volatility or short-sightedness or sheer pettiness, I really don't think that's the case here.
US policymakers have been pushing for war with Iran for almost half a century now, to the point where it really seems like a miracle that we haven't gone to war with them yet. At most, the impeachment/election issues explain the timing, but even that's better explained by the fact that Iran recently discovered a massive cache of oil, just a few weeks before the US assassinated one of their generals.
So for the sake of clarity, here's a list of all the reasons why American politicians really want this war, in order of importance:
Realpolitik
The US wants to establish an American hegemony throughout the Middle East, and Iran is currently our greatest obstacle, both because they fund anti-American groups in other countries (Lebanon, Syria, Yemen), and because they're the largest and most powerful country in the region that's overtly opposed to our interests. They're also allied with Russia and China, which gives us all the more reason to want them out of the way.
Oil
Back in 1953, Iran's government tried to nationalise their country's oil reserves. As a result, the US and Great Britain sponsored a coup, overthrowing the democratically-elected government and replacing it with an oppressive dictatorship willing to serve Anglo-American economic interests. This lasted until 1979, when revolutionaries overthrew the dictatorship and implemented the current system of even-more-oppressive Islamist theocracy. We've been trying to regain access to that oil ever since. (This also ties into realpolitik because in addition to wanting the oil for ourselves, we have an interest in keeping Iran from selling it to our rivals. Can't let those scary Russians get their hands on it!)
Saudi Arabia
The Saudi regime is deeply opposed to Iran, both for sectarian and ideological reasons (Saudi Arabia is a conservative Sunni Muslim monarchy with a majority Arab population, Iran is a leftist Shia Muslim quasi-democracy with a majority Persian population), and simply because they're the two most powerful nation-states in the region. They've been in a Cold War with each other for decades now, competing for power and influence, waging brutal and bloody proxy wars against each other. The US also happens to be extremely supportive of Saudi Arabia (partially because they help enforce American hegemony in the region, partially because they sell us cheap oil and buy our weapons, partially because they keep the value of the American dollar high by forcing European and East Asian nations to use our currency when buying oil from them), and the Saudis have been trying to push us into an overt war with Iran for years now.
Israel
For decades, Iran has been funding anti-Israel groups in the Palestinian territories and in Lebanon, making them one of the greatest threats to Israel's ongoing occupation of the Gaza Strip and the West Bank. And, like Saudi Arabia, Israel has very close ties to the US, to the point that our government considers them our "greatest ally" in the region. Since the Suez Canal Crisis in 1956, Israel has helped American and European powers maintain their interests in the Middle East in exchange for support and protection. Additionally, there are many Jewish interest groups in the US that actively lobby Congress on behalf of Israel. As a result, Israel has had a disproportionate amount of influence over American policy in the Middle East, and they've been pushing for this war for as long as I can remember.
The military-industrial complex
War is good for business, and the US is home to many arms manufacturers and defence contractors and mercenary outfits who stand to profit off this war. And just like the oil companies, those corporations have a great amount of influence over our government as a result of lobbying and some very generous donations to American politicians. There are also quite a few Congressional districts where they employ a good percentage of the population, which gives them even more influence over our electoral system (after all, voters with stable and high-paying jobs at Lockheed Martin or Boeing won't be inclined to support policies that could put those jobs at risk). I don't think this is the main reason for our upcoming war with Iran, but I do think it's certainly an added bonus for defence industry executives and the politicians in bed with them.
Spite
There are a great many political and military officials who simply haven't gotten over the Iran Hostage Crisis back in 1979. They see it as a slight that needs to be punished, an embarrassment to our nation that's all the more humiliating because the perpetrators got away with it. In their view, beating Iran into submission is the only way to prove that the US is still number one and that even the slightest offence against us will be addressed with grossly disproportionate retribution.