WASHINGTON (MarketWatch) — Military spending surged in the third quarter to the fastest rate in five years. So what exactly did the Pentagon spend money on?

The 16% spike in defense spending was driven mainly by higher outlays on services to support military members instead of purchases of tanks, fighter jets and ships.

Outlays on services jumped nearly 17%, with most of the gains coming in a trio of categories known as personnel, weapons and installation support. The military also spent a bit more on ammo, missiles and oil.

Spending on major weapons actually fell slightly.

Although the third-quarter increase was unusually large, defense spending is notoriously volatile and often lumpy. And spending on military services appears to have a pronounced tendency to increase sharply in the July-to-September period.

Sometimes a big increase is merely catchup after several quarters of weak outlays. Military outlays, for instance, rose a meager 0.9% rate in the second quarter after falling by a 4% rate in the first quarter and dropping 11.4% in the 2013 fourth quarter.

What might have also contributed to the spike was a the renewal of military action in the Middle East to counter the growing threat from the Islamic militant group known as ISIS. The Pentagon is using more ammo and missiles and also has to support a larger military presence in the region.

The more conspiratorial minded might be led to believe the Obama administration goosed military spending to try to boost the economy just ahead of the November elections. Without the sharp rise in spending, the U.S. would have grown a lot slower than the preliminary 3.5% growth rate.

Read about 3.5% gain in third-quarter GDP.

Circumstantial evidence suggests administrations from both parties have adopted that tactic in the past.

Military spending, for example, surged 11.9% in the third quarter of 2012 when President Obama ran for reelection. Defense outlays also jumped 17.9% in the third quarter of 2008 under a Bush administration that was supportive of Republican Sen. John McCain’s bid for office.

Similar spikes in defense outlays have occurred in the key election years of 2004, 1994 and 1986.

Yet military spending has also declined ahead of other key elections, most likely because the Defense Department often signs large, long-term contracts on which outlays are spread out. Budget constraints and divided political control also help to curb sharp and sudden changes in military expenditures.

Indeed, big jumps in military spending are almost always preceded or followed by smaller increases or outright declines. Even the most cynical presidents only have the ability to shift spending a bit here and there without changing the overall level of outlays.

So look for defense spending to taper off or decline in the fourth quarter.