In the aftermath of the horrendous murder of women and children in the Mexican state of Sonora, President Donald Trump tweeted on Nov. 5: “If Mexico needs or requests help in cleaning out these monsters, the United States stands ready, willing & able to get involved and do the job quickly and effectively. The great new President of Mexico has made this a big issue, but the cartels have become so large and powerful that you sometimes need an army to defeat an army!”

Not surprisingly, the government of Mexican President Andrés Manuel López Obrador rejected President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE’s “offer” to send troops to Mexico to assist in fighting the drug lords.

Was Trump serious? Or was this yet another impulsive tweet? One never can be sure. Perhaps the president had in mind the Mexican-American War, in which U.S. forces won a smashing victory that resulted in Mexico’s loss of about a third of its territory, comprising nearly all of present-day California, Nevada, New Mexico and Arizona.

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Fighting the drug lords in Mexico likely would result in an exceedingly less positive outcome for the United States than the war that President James K. Polk initiated against Mexico in 1846. On the other hand, it would be far more demanding than the 11-month expedition into Mexico led by Gen. John J. Pershing in 1914-15 to hunt down the bandit Pancho Villa. That operation ended without Villa’s capture, as U.S. forces withdrew across the border in the face of armed resistance by the Mexican Army.

The prospects for a new U.S. incursion into Mexico seem no brighter.

To begin with, a Mexican operation would be an exceedingly costly affair, even if U.S. forces do not have to face off against Mexican troops. Yet, there is no guarantee that Obrador, a veteran leftist whose political views are the polar opposites of Donald Trump’s and who long has harbored resentment against the United States, would hesitate to send his army to confront U.S. forces.

Even if U.S. forces did not have to confront Mexican troops, an operation against the drug lords would be an excessively demanding, costly and long-term affair, probably far longer than Polk’s Mexican-American War that lasted two years.

There can be little doubt that the drug lords would fight back with all the firepower at their command. And they do command considerable firepower, including high-technology weaponry. Moreover, U.S. forces would not be fighting set-piece battles. Instead, the drug lords could be expected to fight a guerrilla war, with complete knowledge of their own home turf. America’s recent experiences in Afghanistan and Iraq do not bode well for its fighting yet another such war.

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The cost of such an operation not only would be exceedingly high, but it would draw off funds from Department of Defense programs that are urgently required to cope with growing threats from China, Russia and possibly a nuclear-armed Iran. The current deployment of active and reserve forces to the border is costing between $850,000 and $1 million annually for every active soldier, and nearly as much for every reservist. That operation is likely to pass the billion-dollar mark, with no end in sight. Those U.S. troops stationed along the border are not facing active, armed resistance from migrants, as would certainly be the case if they confronted the drug lords.

Finally, it is no secret that U.S. forces already are stretched thin; that is one reason President Trump wants to bring them home. The last thing he should want is another “endless war.” But if he were to send troops into Mexico, that is precisely what he can expect to get.

With his mind consumed by impeachment, and in light of the massive numbers of tweets that emanate from his computer, it is likely that the president already has forgotten his offer to his Mexican counterpart — and both the United States and Mexico are the better for it.

Dov S. Zakheim is a senior adviser at the Center for Strategic and International Studies and vice chairman of the board for the Foreign Policy Research Institute. He was under secretary of Defense (comptroller) and chief financial officer for the Department of Defense from 2001 to 2004 and a deputy under secretary of Defense from 1985 to 1987.