In May, President Trump pulled the United States out of the joint comprehensive plan of action (JCPOA). Many saw this as an aggressive move and Trump was attacked by most of the media. Europe did not follow the United States action however it might be in their best interests.
One problem with the JCPOA is that restrictions on producing nuclear weapons are mainly temporary and will expire in 10-15 years. While some might argue that after 10-15 years the JCPOA will be reevaluated and sanctions could then be placed on Iran, they would surely have saved up enough resources to quickly produce a nuclear warhead within a few years or even months. Another problem with the JCPOA is that it does not stop Iran's ballistic missile program. This means Iran is likely attempting to develop missiles capable of striking the United States so that they could deter any US attack on them. Arguably the biggest problem with the JCPOA is that Iran has used its oil revenues to support its proxies in wars all over the middle east. For example, it has been reported by Israel that Iran has approximately 80,000 or so fighters in Syria made up of lots of proxy groups. Many of these fighters are stationed near the border with Israel and they have fired dozens of missiles into Israel. It also supports Houthi rebels in Yemen who have attacked Saudi oil tankers destined for the West.
If Trump had not pulled out of the JCPOA and the West waited, then it would have been highly likely that in a few years time Iran would be able to cut off some of the worlds oil supply at any moment as they would control the Bab el-Mandab and Strait of Hormuz choke points after a successful war in Yemen; funded by the lack of sanctions on their oil supply. They would also likely have developed ballistic missiles that were capable of hitting their enemies throughout the middle east. Iran would leverage this power against the West and feel free to expand their influence throughout the Middle East.
Currently, Trump is placing sanctions on Iran. This has caused a run on black markets for the Iranian Rial. It recently fell to its lowest value ever roughly 110,000 Rial to 1 US dollar. This is due to Iranian civilians selling their Rials and buying dollars to try and preserve their wealth. This has caused massive shortages of imported items and inflation is predicted to reach at least 50% this year. Trump is hesitant to completely sanction Iran due to the fact that they produce over 2 million barrels per day of oil and he is worried a rising oil price will affect the US economy. However many economists now believe that any loss of economic growth due to reduced disposable income for consumers will be offset by an increase in oil projects and profits for oil companies. If Trump sanctions Iran completely we will likely see mass uprisings similar to the ones that brought the Islamic Revolution in 1979. This will be the most peaceful way to prevent the West from being held hostage to Iran.
The only other way to curb Iran is to attack Iran. An attack on Iran would only be justifiable if it was a targeted attack against nuclear and missile facilities. The US Navy would need to be guarding the Straits of Hormuz and Bab el-Mandab to ensure oil continued to flow to the West. An attack on Iran could lead to major assaults throughout the Middle East from Iran's proxies. Should Iran retaliate, an attack on Saudi Arabia via Yemen would be highly likely along with the potential for a new war between Israel and Hamas. Iran has roughly 500,000 active duty soldiers, Saudi Arabia and the UAE have around 400,000 between them. This means America would not have to commit regular soldiers as the numbers are already there and using overwhelming air support Iran would be able to be halted. Using air power would be significantly more effective as Iran is a conventional army and so are easily distinguished from civilians than the Taliban ever have been. However, this is only an option as a last resort against Iran as there would be an immense death toll on all sides and there is a chance America would be forced to commit regular soldiers should any Iranian retaliation not play out as planned. That would be a disaster and almost guarantee a lost war as the public can not tolerate casualties.