Iranian President Hassan Rouhani and US President Donald Trump. Sergei Chirikov/Pool via REUTERS; GOL/Capital Pictures/MediaPunch/AP

The US could be near the brink of war with Iran again after months of rising tensions.

The Pentagon on Thursday announced the US had killed a top Iranian general, Qassem Soleimani.

Iran's top diplomat described Soleimani's death as an act of "international terrorism" by the US.

A war with Iran would likely be geopolitically and economically disastrous while further destabilizing a region that has been consumed by conflict for years.

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Tensions between the US and Iran have reached historic heights in recent months, prompting fears of a military confrontation that could escalate into all-out war.

Here's a breakdown of what's going on, how we got here, and what the stakes are.

What's going on with Iran?

In May, the US deployed military assets to the Middle East to counter threats from Iran. This was around the same time US sanctions meant to choke out Iran's oil revenue went into full effect.

Within weeks, oil tankers in the region were attacked, which the US blamed on Iran. The US said Iran used naval mines to sabotage the tankers. Iran also seized oil tankers, which further increased tensions.

In late June, Iran shot down a US Navy drone, which nearly prompted a military response from President Donald Trump. Trump called off the retaliatory strike at the last minute, however, stating it would not have been proportionate to the downing of an unmanned aircraft.

In the time since, the US has continued to issue economic sanctions as Trump has occasionally flirted with the idea of holding talks with Iran.

Read more: Iran could be risking war over the Saudi oil fields because it deliberately wants to spike up the price of oil

Iran in recent months took several major steps away from the 2015 nuclear deal orchestrated by the Obama administration, raising concerns among European countries who were also signatories to the crumbling agreement. Iran has taken such actions in an effort to gain leverage and relief from US sanctions, but has not had much success.

For a brief window, it seemed possible that Trump and Iranian President Hassan Rouhani might meet to find a way to end the stalemate, but Iran has ruled out any talks unless the US lifts sanctions and returns to the 2015 nuclear deal. Trump pulled the US out of the deal in May 2018, and US-Iran relations have deteriorated ever since.

The situation escalated significantly following attacks on two major oil facilities in Saudi Arabia on September 14, which disrupted the global oil supply. Iran-backed Houthi rebels in Yemen claimed responsibility, but both the US and Saudi Arabia have implicated Tehran.

Top Republicans in Congress and key US officials, including Secretary of State Mike Pompeo, have described the incident as an act of war by Iran against Saudi Arabia. Former US officials and foreign policy experts say the signs point to Iran as the culprit, but neither Washington nor Riyadh have presented definitive evidence that Tehran is responsible for the oil field attacks.

Shortly after the incident, Trump tweeted that the US was "locked and loaded," indicating that a US military response could be on the horizon. He's since walked back on that somewhat, and said he's not interested in going to war. Trump has also issued new wave of sanctions against Iran in recent months.

But Trump is surrounded by advisers and politicians who are hawkish and fiercely anti-Iran, which is raising anxieties in Washington and beyond about the potential for conflict.

Iran's foreign minister Javad Zarif on in September warned that a US or Saudi military strike against his country would lead to "all-out war."

"I make a very serious statement about defending our country. I am making a very serious statement that we don't want to engage in a military confrontation," Zarif told CNN. "We won't blink to defend our territory."

In late December, a rocket attack killed a US contractor and injured four US service members at a base in Kirkuk, which is in the northeastern part of Iraq. The US has blamed the deadly attack on Kataeb Hezbollah, an Iran-backed militia. The US retaliated with airstrikes that killed dozens of the militia's fighters. Subsequently, the US embassy in Baghdad was consumed by violent protests, prompting the US to send about 100 more Marines to the embassy for protection.

Trump explicitly blamed Iran for the violent protests at the embassy, and threatened Tehran in a tweet: "Iran will be held fully responsible for lives lost, or damage incurred, at any of our facilities. They will pay a very BIG PRICE! This is not a Warning, it is a Threat. Happy New Year!"

On the morning of January 2, Defense Secretary Mark Esper warned the US might conduct preemptive strikes on Iranian-backed militias. Esper said: "If we get word of attacks, we will take preemptive action as well to protect American forces, protect American lives. The game has changed."

Later on the same day, the Pentagon confirmed that the US military, acting on the orders of the president, killed top Iranian general Qassem Soleimani. "The US military has taken decisive defensive action to protect US personnel abroad by killing Qasem Soleimani, the head of the Iranian Revolutionary Guard Corps-Quds Force," the Pentagon said in a statement.

Zarif described the Soleimani's death as an act of "international terrorism."

How did we get here?

The US and Iran have a complicated history and have been adversaries for decades, encapsulated in the oft-repeated "Death to America" chants from Iranian leaders.

In many ways, the modern US-Iran relationship began via a CIA-orchestrated coup in the 1950s that placed a pro-American monarch - Shah Mohammad Reza Pahlavi - in charge of the Middle Eastern country. The shah was overthrown in the 1979 Islamic Revolution, an uprising that shook the foundations of the Muslim world and led to the infamous hostage crisis at the US embassy in Tehran that continues to be a touchy subject in Washington.

Read more: CIA predicted the Iran crisis that spiraled out of Trump pulling the US from the 2015 nuclear deal

After years of animosity, former President Barack Obama sought to improve relations with Iran via diplomacy. Obama's administration orchestrated the landmark pact known as the Iran nuclear deal, which was finalized in July 2015 and hoped to prevent Iran from obtaining nuclear weapons in exchange for the easing of economic sanctions.

Critics of the deal said it didn't go far enough to bar Iran from building nuclear weapons and that Tehran could not be trusted. Along these lines, Trump withdrew the US from the deal in May 2018 despite having no evidence Iran was violating its terms. This move put Washington at odds with key allies, and the already contentious US-Iran relationship took a turn for the worse.

The situation was hardly improved after Trump in April designated Iran's elite Revolutionary Guard Corps as a foreign terror organization. This prompted Iranian leaders to warn that any action taken against the country would lead to "a reciprocal action." The Trump administration in April also announced it would move to block all countries from buying Iran's oil on top of the sanctions already crippling the Iranian economy.

The US and Iran have also been working against one another in the ongoing war in Yemen, where the US-backed Saudi-led coalition is fighting against the Iran-backed Houthi rebels. And in the ongoing Syria conflict, Iran and its proxies have supported Syrian President Bashar Assad, whose forces Trump has launched military strikes against.

What are the stakes?

A war with Iran would potentially be more calamitous than the US invasion of Iraq in 2003, which led to hundreds of thousands of deaths, bogged the US down in a costly and lengthy war, and helped catalyze the rise of the Islamic State group.

Iran has a population of about 82 million people, and its military is ranked as the 14th most powerful. According to recent estimates, Iran has 523,000 active military personnel in addition to 250,000 reserve personnel.

Comparatively, Iraq had a population of about 25 million people, and the Iraqi military had fewer than 450,000 personnel when the US invaded over a decade ago.

Iran is also much bigger than Iraq geographically. It has 591,000 square miles of land versus Iraq's 168,000 square miles, and its influence has grown as the power of its rival Iraq collapsed in the wake of the US war there.

If the US launched an attack against Iran, it would also reverberate across the Middle East. Iran has proxies throughout the region and is allied with militant groups, such as Hezbollah in Lebanon. A revised Pentagon estimate released in April found Iranian proxy forces killed at least 608 US troops in Iraq between 2003 and 2011.

Read more: How the Trump administration got into a showdown with Iran that could lead to war

Moreover, Iran shares a border with a number of countries the US considers allies and has a military presence in, including Turkey, Iraq, and Afghanistan. None of these countries are especially stable at the moment, as they all continue to deal with ongoing conflicts and their consequences (including millions of displaced people).

In terms of other geopolitical blowback, Iran is allied with Russia and China, and it's unclear how these major powers might react if conflict breaks out. Key US allies such as Israel and Saudi Arabia, which are adversaries of Iran and just a stone's throw away from it, would also likely get sucked into a US-Iran war.

A war with Iran could also be extraordinarily disruptive economically, given it borders the Strait of Hormuz, a narrow route that roughly one-third of the world's oil-tanker traffic travels through. Experts have predicted that if the route were blocked, it would quickly lead to a 30% drop in daily global oil exports, and prices would rapidly go up, The Washington Post reported.

Iran's forces would likely be defeated by the US but could exact a heavy toll with cruise missiles, naval mines, and fighter jets. Any troops that survive could blend into the population and lead a brutal insurgency against the US occupation force. That was the scenario that unfolded for the US in Iraq, a country one-third the size of Iran, and proved to be an insurmountable challenge.

In short, though the US has a military that is consistently ranked the most powerful, evidence suggests a war with Iran would be devastating in myriad ways.

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