A top US general picked apart Donald Trump's ISIS policy

Republican presidential hopeful Donald Trump's vague strategy to combat Islamic State extremists in the Middle East isn't very likely to work.

A top US general who commanded troops in Iraq from 2008 to 2010 explained the holes in Trump's strategy, The Washington Post reports .

Earlier this week, billionaire real-estate developer Trump said he would send American soldiers to Iraq to target the Islamic State (also known as ISIS, ISIL, and Daesh), adding that the US would "knock the hell out of them" in "every place where they have oil."

"They have great money because they have oil," Trump said. "... I would knock out the source of their wealth, the primary sources of their wealth, which is oil. And in order to do that, you would have to put boots on the ground. I would knock the hell out of them, but I'd put a ring around it and I'd take the oil for our country."

When reporters at a briefing on Wednesday asked outgoing Army Chief of Staff Gen. Ray Odierno if he disagreed with Trump's strategy, he said: "I do, I do. Right now, I do."

Trump's strategy is rather simplistic and shortsighted, Odierno said, noting that other countries in the Middle East also need to be part of the solution.

"The problem we've had is we've had outcomes, but they've been only short-term outcomes because we haven't properly looked at the political and economic side of it," Odierno said at the press briefing. "It has got to be three that come together. And if you don't do that, it will not solve the problem, and that is what I continue to look at."

Trump's understanding of ISIS' finances doesn't seem to be very complete either. While ISIS does make millions of dollars from oil, the biggest slice of its budget likely comes from taxing the "Caliphate's" captive population. And this source of wealth is harder for international actors to control.

Analysts at the nonprofit RAND Corporation estimated that in 2014, ISIS raked in $600 million from extortion and taxation, $500 million in money stolen from Iraqi banks, and $100 million from oil.

The US has been targeting ISIS' oil revenues and donations from "overseas benefactors" in an effort to cripple the terror group, but that still leaves ISIS free to collect taxes, as The Wall Street Journal reported in June.

"ISIS makes most of its money from plunder,"Jonathan Schanzer, vice president for research at the Foundation for Defense of Democracies, told Business Insider in May. "We're seeing that over and over again. They go from one town to the next and knock over a bank or several banks and go house to house and extract whatever is of value."

"It's a racket," Schanzer said. "And that's how ISIS continues to survive and thrive."

Trump's oil-focused strategy also doesn't address the tactics ISIS has been using to entrench itself in communities and establish a long-term presence in the Middle East.

ISIS has claimed territory across Syria and Iraq, forcing those who stay in those areas to pay taxes to the jihadist militants, sometimes in return for government-like services.

For example, in Palmyra, the ancient Syrian city Islamic State militants overran in May, ISIS fixed the city's power plant, turned on the water pumps, and handed out free bread to residents.

This isn't the first time that Trump has talked a big game on ISIS.

In June, he said: "Nobody would be tougher on ISIS than Donald Trump, nobody. I will find the General Patton or I will find General MacArthur. I will find the right guy. I will find the guy that's going to take that military and make it really work."