The national average of the price of a gallon of gasoline now stands at $4.05. It broke through the $4-a-gallon barrier today after spending roughly a week hovering just below it. This isn't really a major change for consumers (it is only the difference of a few pennies), but it is a big psychological barrier that has now been breached. The only time gas prices have been higher in the past decade was at the start of Russia's invasion of Ukraine (when not only gas prices spiked but the inflation rate skyrocketed as well). Then, the average U.S. price of gas rose to $5.00 per gallon before falling back again.

The future trajectory of gas prices this time around is still an open question. Depending on what happens next, it could continue to climb, it could flatten out and peak, or it could even start the long, slow slide back down again. The biggest factor in all of this is whether Donald Trump decides to send American soldiers to invade Iranian soil.

As usual, nobody knows what Trump is going to do. Previously (during his second term), when he has created a big buildup of military power, he has then used it. So his buildup of troops in the region could mean that he is already committed to some sort of ground assault. Or maybe it's all just a bluff (or he will change his mind). He certainly has been sounding very desperate to figure out some way out of the war of choice he started, with some sort of agreement with the Iranians that could stop the fighting.

Sending U.S. troops into Iran could mean a number of different things. The most serious action is probably not even on the table at this point, because even with a total of 50,000 American troops in the Middle East region, it wouldn't be enough to launch a full-scale invasion of Iran on the order of what we did in Iraq -- with the intent to take the country over and militarily rule it for the near future. This is what it would likely take to effect true regime change in the country. But even 50,000 soldiers is nowhere near enough to achieve that objective, so that is not very likely to happen any time soon.

There are three other smaller military objectives that might happen instead, though. Trump has now moved over 4,000 Marines and over 1,000 paratroopers of the 82nd Airborne Division into the region. That is a much more limited number of troops, which means what they could be expected to accomplish is also a lot more limited.

Trump has openly spoken about the most obvious target: Kharg Island. This is a small island off the coast of Iran that the country uses to ship 90 percent of its oil. The island has basically become a giant loading dock or pumping station for Iranian oil. Oil tankers pull up to it, fill up, and then sail away. It is further up the Persian Gulf than the Strait of Hormuz, so any ship that uses it still has to traverse the Strait to get to the rest of the world.

The U.S. has already bombed military installations on the island, but so far hasn't destroyed the oil infrastructure. If the Marines landed and took the island, it would cut off Iran's ability to ship their oil to the world. Doing so would not mean that Donald Trump would then be in charge of Iran's oil -- because the flow of oil to the island from the rest of Iran would stop. This would have another secondary effect, though, because removing Iranian oil from the world's supply would almost guarantee that the price of oil rises higher (it is currently in the $105-$115 per barrel range). It would hold hostage Iran's ability to make money, but it would also hurt Americans at home, at the local gas pump. Gasoline could rise to $4.50 a gallon or even higher, in other words. So this sword cuts both ways.

There is another big objective that U.S. troops could try to achieve as well: securing and removing the nuclear material that Iran still has. They've got enough enriched uranium to make roughly 10 nuclear devices, but it is all stored underground and may be buried in the rubble of a facility that we've already bombed multiple times. This all means securing and removing it is not going to be easy. And any U.S. troops sent in to attempt this would be largely on their own. Paratroopers could land and fight their way to securing a patch of land, but it would be a little island of territory surrounded by the rest of Iraq. These soldiers would be subject to attacks from the Iranian military. And digging the nuclear material out could take many weeks or even many months.

The third objective would be for American troops to invade the Iranian coastline of the Strait of Hormuz. They could attack all the towns and villages along this stretch of coast and perhaps move inland a number of miles, in an attempt to prevent Iranian attacks on oil tankers moving through the Strait. If American Marines held all the ports, then it would be impossible for Iran to launch boats or mine the Strait. It would also be much more difficult for them to launch missiles at tankers, but not impossible (depending on the range and accuracy of the missiles).

But to do this, the Marines would have to take all this territory and defend it. And again, the Iranian military will be attacking them from the inland side of this buffer zone. Securing the Strait might be able to be done, but it would have to be a long-term accomplishment, not just a quick raid (because securing it for a week or any other short period before pulling out wouldn't really accomplish the overall goal of securing the Strait).

Trump and his military advisors are reportedly leaning towards a quick raid scenario (using special forces, not regular troops), but it's hard to see what this could accomplish. We could go in and destroy Kharg Island and then pull out, but we could probably accomplish that from the air if we really wanted to. We could raid the coastal towns and destroy all the boats we can find, but if we left immediately afterwards then the Iranians could still eventually manage to launch attacks into the Strait. Securing and removing the nuclear material might have been possible if it wasn't buried far underground under tons of rubble, but a quick raid is now not going to have any reasonable chance of success.

Having American troops stay longer than a quick raid will expose them to attack. So far, this war has been notable for the low level of casualties on the American side (only 13 U.S. soldiers have died in the conflict so far, after a whole month), which is due mostly to the remote nature of how we've been waging it. But putting troops into harm's way is almost guaranteed to send the body count far higher. The longer they have to stay on Iranian soil, the worse this problem is likely to get.

If American ground troops do invade Iranian soil, the world's oil markets are probably going to react by sending the price per barrel higher. How much higher is anyone's guess, but it will all be added on top of the already $4-a-gallon prices American consumers are now paying. That is a political problem for Trump and for his fellow Republicans. The public is already strongly against the concept of sending in troops, and if they do go in and a number of them get killed, public opinion is likely going to get even worse for Trump.

Nobody knows what Trump is going to do next (nobody ever does, with him). Since he has moved all those troops into place, he might just decide to go ahead and use them. Or he might find some way to wiggle out of everything and declare victory and go home. But if the troops do go in -- no matter what their objective is -- perhaps in the near future we will all look back at today and say: "Remember when gas was only four bucks a gallon?"

-- Chris Weigant

Follow Chris on Twitter: @ChrisWeigant