It's a long and complicated issue that goes back literally thousands of years to Cain & Able. I'm not going to pretend to know all the ins and outs of it.
But there are a couple of things: It's the islam wiping out the jews/christianity thing that has been going on for millenia.
Next, Iran since the overthrow of the Shaw has used religion as a basis to try to control it's people and influence other countries. It wants to be in charge of the Middle East. There is also the "our version is Islam is the right version and everyone else is a heretic and therefore should die" mentality at work because...duh...power and control. Iran controls Hezbollah in the West Bank. Iran also controls Hezbollah which has kept Lebanon a dungheap for the last 40 years. Iran has propped up Syria in exchange for influence and control in Syria. The list continues with Somalia...etc.
The US is getting involved because of Bolton's war mongering and belief in war as a solution (extremely simplified overview, it's complicated). Add in Putin's control of Trump and using Trump to create chaos in other areas keeps us weaker and not looking at the things Putin/Russia is doing to hurt democracy around the world and give Russia influence that it lost after the breakup of the Soviet Block/Warsaw Pact. Again, we could go on for days, but Russia is behind many of the issues going on in the world today. Brexit was pushed and influenced by Russia. Been proven. The UK leaving the EU makes for a weaker EU and a weaker UK. A weaker EU and weak UK means Russia can work on influencing other countries and especially the former Soviet block countries. The rise of nationalism in countries like Poland and Hungary are also due to the direct influence of Putin's Russia. he goal is to use Nationalism to divide and weaken the countries and separate them from the EU. That allows Russia to get the candidates they want to a position of power and influence and essentially Russian control. The fall of democracy in Turkey with Erdogan's coup was perfectly executed using Putin's textbook. It was a thing of beauty and slowly, evidence of this is coming out. You also see it in the rise of nationalism in the US. With the US divided and antagonizing it's allies, the resistance to Russia grows weaker. The US is fighting itself and not paying attention to what others around the world are doing. The Iran issue is just an extension of that. If the US is mired in another conflict with another shitehole country, attention won't be focused on other areas and issues.
If you look at Steve Bannon's influence in the White House, you will see a lot of the why. It's really whacked but it's there. Bannon is a white nationalist/supremasist. He's also militantly pro-Israel to the point of pushing the anti-muslim movement. Bannon believes Iran, China, and Turkey are teaming up to create a new 'axis' to challenge the US (
link) and you see the policies of the Trump administration accordingly.
I need to stop as I have to get work done. But the answer is there is no simple answer. It's complicated and a ton of nuances to the whole mess.