Brent crude crossed $100 a barrel Wednesday as the expanding conflict with Iran stoked fear in energy markets.

The immediate explanation is war, but the longer-term explanation may be more complicated.

For decades, much of American economic policy has focused on the financial side of the economy.

But financial markets ultimately sit on top of a physical economy.

In a healthy economy, money has to eventually become something productive.

And in the oil and gas business specifically, maintaining productive capacity requires enormous amounts of continuing investment.

The International Energy Agency recently examined production from roughly 15,000 oil and gas fields around the world and reached a concerning conclusion.

Without continued investment, global oil production from existing sources would fall by about 8% per year over the next decade.

In other words, enormous amounts of capital have to be invested just to maintain the productive capacity we already have.

Making matters worse, addressing shortages in that capacity is not a quick fix.

Producing more energy isn’t simply a matter of flipping a switch. New infrastructure across the sector can take years to build.

And that exposes the limits of treating economic problems primarily in financial terms.

The Fed can move interest rates and Treasury can intervene in currency markets, but none of those actions immediately produces, for example, another barrel of oil.

And the principle extends far beyond energy.

Productive assets have to be maintained and eventually replaced.

Financial markets are highly useful and an important part of a modern economy.

But ultimately, an economy’s ability to produce goods and services depends on productive infrastructure and people.

The return of $100 oil is providing an expensive reminder of that distinction.

America has become extraordinarily sophisticated at moving money.

The harder question is whether enough of that money has been used to build and maintain the things the country actually needs.