ZANGABAD, Afghanistan  American commanders are planning to cut off the Taliban’s main source of money, the country’s multimillion-dollar opium crop, by pouring thousands of troops into the three provinces that bankroll much of the group’s operations.

The plan to send 20,000 Marines and soldiers into Helmand, Kandahar and Zabul Provinces this summer promises weeks and perhaps months of heavy fighting, since American officers expect the Taliban to vigorously defend what makes up the economic engine for the insurgency. The additional troops, the centerpiece of President Obama’s effort to reverse the course of the seven-year war, will roughly double the number already in southern Afghanistan. The troops already fighting there are universally seen as overwhelmed. In many cases, the Americans will be pushing into areas where few or no troops have been before.

Through extortion and taxation, the Taliban are believed to reap as much as $300 million a year from Afghanistan’s opium trade, which now makes up 90 percent of the world’s total. That is enough, the Americans say, to sustain all of the Taliban’s military operations in southern Afghanistan for an entire year.

“Opium is their financial engine,” said Brig. Gen. John Nicholson, the deputy commander of NATO forces in southern Afghanistan. “That is why we think he will fight for these areas.”