The biggest energy story in the world right now is the collapse of global oil prices. And for major producers like Russia or Iran or even Saudi Arabia, that's unsettling news.

How bad is it? One good place to start is with this excellent chart from The Economist looking at the "break-even" points for various countries. These are all nations that depend heavily on oil revenues for their national budgets — so when the price falls below the break-even point, the government starts running a deficit:

At the start of 2014, oil prices were already below the break-even point for Iran, Venezuela, Nigeria, and Iraq. But now, as prices sink below $90 per barrel, they're falling below the break-even point for Libya, Russia, and Saudi Arabia.

Below is more detail on three key countries: Russia, Iran, and Saudi Arabia. At the moment, it's looking like Russia's economy could be in serious trouble, Iran's is in flux — and Saudi Arabia seems far more sanguine about the situation.

How falling oil prices could crush Russia's economy

Russia's situation is getting most of the attention these days. The country was already suffering from weak growth — on pace to expand just 0.4 percent in 2014. That was partly due to the ongoing Ukrainian crisis (Ukraine's economy is tightly intertwined with Russia's) and partly due to Western sanctions.

Russia had been planning for $100-per-barrel oil in its budget for 2015

Now the plunge in global oil prices is putting even further strain on the nation's economy. Oil revenues account for roughly 45 percent of Russia's budget, and the government's spending plans for 2015 had assumed that prices would stay in the $100-per-barrel range. If oil continues to sink below that, the country will either have to draw down from its $74 billion foreign-exchange reserves or cut back on planned spending — something that Russian President Vladimir Putin suggested was possible on Tuesday.

The economic impact could be deeper still: As Brookings Institution economist Clifford Gaddy explains, Russia's oil and gas industry helps prop up the nation's ailing economy in all sorts of informal ways. For instance, oil companies face pressure to purchase machinery from Russian factories that support jobs in key areas around the country — even if those factories aren't the most efficient out there. It's a system that has been semi-directed by Putin's government and has been crucial to maintaining stability.

Now that system will come under heavy pressure if prices keep falling.

"It's not hard to figure out how falling oil prices will affect Russia's formal budget," Gaddy says. "What's much more difficult is to figure out how falling prices will affect that informal system." In a crunch like this, the Russian government could struggle to figure out how to make sure that oil and gas wealth still gets distributed around the country.

The crash in oil prices in the 1980s famously battered the Soviet Union and helped contribute to its ultimate collapse in 1991. Gaddy points out that Putin has spent a lot of time studying that previous episode — and has likely prepared for another price crash. Whether he can deal with it successfully, however, is another question.

In the meantime, the Los Angeles Times recently took a look at the foreign policy ramifications of plunging oil prices: "The economic pressure isn't expected to change Putin's aggressive efforts to retain strong influence over Ukraine, which he considers nonnegotiable." But, the Times noted, it's "causing strains in [Putin's] relations with the Russian elite and business establishment, two pillars of his political support."

Iran's economy was rebounding — before the price drop

Iran's economy had recently started to rebound after years of recession (due in part to heavy Western sanctions). The International Monetary Fund had been projecting that the country was on track to grow 1.5 percent this year and 2.3 percent next year.

Iran was on pace to grow again this year — until the price collapse hit

But that was before oil prices started to drop — a potentially precarious situation.

One big problem for Iran is that it also needs oil prices well north of $100 per barrel to balance its budget, especially since Western sanctions have made it much harder to export crude. And if oil prices keep falling, the Iranian government may need to make up revenues elsewhere — say, by paring back fuel subsidies for its citizens (always an unpopular move, at least in the short term).

The Times also noted that the oil plunge could alter Iran's foreign-policy stance: "The new economic strain also undercuts Tehran's strategy for dealing with world powers in talks over Iran's nuclear program. Iran's economic resurgence had enabled Iranian officials to claim they could get by even if the talks collapsed without providing further relief from tough international sanctions."

Those ongoing negotiations are a wild card here. The United States, EU, and Iran are trying to hammer out a deal on the latter's nuclear program before a deadline in late November. If there actually is a deal — and if the the US and Europe offer some sort of sanctions relief — that could help bolster Iran's economy. Though it could also push oil prices down further as Iranian oil comes back to the global market.

Saudi Arabia, meanwhile, seems more optimistic

Meanwhile, oil prices have now dropped below Saudi Arabia's break-even point — around $93 per barrel. But, so far, many the country's leaders sound a little more confident that they can survive the hit.

Right now, Saudi Arabia is suggesting that they're fine letting oil prices slide

In theory, Saudi Arabia could respond by trying to cut back on its own oil production in order to prop up global prices. (The country is such a massive oil producer that it has a lot of sway here.) But for the time being, the Saudis are signaling that they're prepared for low prices — possibly even a year or two with oil at $80 per barrel.

One possible reason is that Saudi Arabia has learned its lesson from the mid-1980s — a time when global oil prices were plummeting and the kingdom tried to cut back on domestic production in the hopes that other OPEC nations would follow. No one else joined in, global oil prices kept declining anyway and Saudi Arabia suffered massive budget deficits as a result. So, this time, they appear more reluctant to cut back drastically.

And there's another angle here: Officials in Saudi Arabia have signaled that lower prices (say, $80 per barrel or lower) could make some shale-oil producers in the United States unprofitable and force the US to cut back on production. Saudi Arabia would be fine with that, since its oil tends to be cheaper to produce than most shale oil projects in the United States. That US cutback, in turn, could help stabilize prices.

One question is how far Saudi Arabia can let prices fall without incurring too much domestic pain. In September, the International Monetary Fund warned that Saudi Arabia would run a deficit of roughly 1.4 percent in 2015 — not least since the country has been ramping up big infrastructure projects and doling out foreign aid around the Middle East of late. That shortfall would force the country to start drawing down its massive foreign-exchange reserves.

So far, Saudi Arabia seems prepared to wait it out. According to the Financial Times, even if oil prices stayed at $80 per barrel for a year, the country would only need to draw down about $10 or $20 billion of its $750 billion in foreign-exchange reserves. Still, a few observers are worried about the longer run. On Tuesday, Saudi prince al-Waleed bin Talal published an open letter to the country's oil minister warning that "our country faces the danger of continuing to depend almost entirely on oil."

Further reading

Here's an overview of why global oil prices are plummeting — a combination of weaker demand in places like China and a boom in production in the United States and Libya.

Here's a look at which US states get hurt by a fall in prices. Wyoming, Oklahoma, and North Dakota are the most reliant on drilling and top the list. Oil producing states that re more diversified — like California — don't get pinched nearly as sharply.