Trump avoids specifics in Afghan strategy but vows, 'Attack we will' The president addressed the nation about his Afghanistan strategy Monday night.

 -- President Donald Trump announced on Monday night his administration's plans to continue the engagement of the United States military in Afghanistan — a strategy to combat the influence of the Taliban and an ISIS affiliate in the country, forgoing a formal timetable and instead relying on "conditions on the ground" to guide U.S. activities.

"We must acknowledge the reality I'm here to talk about tonight, that nearly 16 years after the Sept. 11 attacks, after the extraordinary sacrifice of blood and treasure, the American people are weary of war without victory," said Trump in an address from Virginia's Joint Base Myer–Henderson Hall.

His announcement follows meetings with military advisers and his national security team at Camp David on Saturday. In June he gave Secretary of Defense James Mattis the authority to set troops levels in Afghanistan, after providing him with similar authority in Iraq and Syria.

Though Trump avoided specifying an increase in the number of service members, Mattis indicated that the U.S. would be increasing its involvement.

"I have directed the chairman of the Joint Chiefs of Staff to make preparations to carry out the president's strategy," Mattis said Monday in a statement from Jordan, where he is traveling this week.

"I will be in consultation with the secretary-general of NATO and our allies — several of which have also committed to increasing their troop numbers," he added. "Together, we will assist the Afghan security forces to destroy the terrorist hub."

Afghan President Ashraf Ghani said he was "grateful" for Trump's "affirmation of support for our efforts to achieve self-reliance and for our joint struggle to rid the region of the threat of terrorism.

"The U.S.-Afghan partnership is stronger than ever in overcoming the threat of terrorism that threatens us all," he said.

Though deepening of U.S. participation is a reversal of the position he held before his bid for the presidency, Trump has demonstrated a willingness to engage militarily in the region through the first seven months of his presidency.

"My original instinct was to pull out, and historically I like following my instincts," said Trump Monday night. "But all of my life, I heard that decisions are much different when you sit behind the desk in the Oval Office."

Despite official combat operations ceasing in 2014, the U.S. continues to guide and train the Afghan military and in April dropped a 22,000-pound "mother of all bombs" on ISIS-occupied caves there.

About 8,400 American troops are stationed in Afghanistan in an advisory capacity. Several thousand U.S. personnel are engaged in counterterrorism operations against al-Qaeda and ISIS-Khorasan, the Islamic State affiliate in Afghanistan.

Top U.S. military officials, including Mattis, support sending as many as 4,000 additional troops as part of a broader revamp of regional strategy, though Trump continued to tout the advantages of secrecy — a position he took during last year's presidential campaign.

"We will not talk about numbers of troops or our plans for further military activities," Trump said.

"America's enemies must never know our plans or believe they can wait us out," he added. "I will not say when we are going to attack, but attack we will."

In February, Army Gen. John Nicholson, the top U.S. official leading the international coalition in Afghanistan, told the Senate Armed Services Committee that the mission had a "shortfall of a few thousand" troops.

In his first formal address since his speech in February to a joint session of Congress, Trump commented on the role he expects other nations in the region to play, placing emphasis on the actions of Pakistan, which he accused of "harbor[ing] terrorists."

"We can no longer be silent about Pakistan's safe havens for terrorist organizations, the Taliban and other groups that pose a threat to the region and beyond," he said. "Pakistan has much to gain from partnering with our effort in Afghanistan."

While Trump promised Monday to support the armed forces with "every weapon to apply swift, decisive and overwhelming force," he also said that the U.S. commitment was "not a blank check."

"The government of Afghanistan must carry their share of the military, political and economic burden," Trump said. "The American people expect to see real reforms, real progress and real results. Our patience is not unlimited."

"Afghans will secure and build their own nation and define their own future," he added. "We want them to succeed, but we will no longer use American military might to construct democracies in faraway lands or try to rebuild other countries in our own image. Those days are over."

In a statement released Monday night, Secretary of State Rex Tillerson said the new strategy signals clear support for the Afghan people and government.

"We will continue to support the Afghan government and security forces in their fight against terrorists and prevent the re-establishment of safe havens in the country," he said. "Our new strategy breaks from previous approaches that set artificial calendar-based deadlines. We are making clear to the Taliban that they will not win on the battlefield. The Taliban has a path to peace and political legitimacy through a negotiated political settlement to end the war."

In response to Trump's remarks, the Taliban said in a statement, "It looks like the U.S. still doesn't want to put an end to its longest war ... If American leaders keep following their war strategy, we will keep fighting them with high spirit and commitment until one American soldier is left in our country."

Trump's decision to increase the U.S. troop level in Afghanistan contrasts sharply with his position from as early as 2012, four years before his election, when he said with frequency on social media that the U.S. should "get out of Afghanistan" and that it has "wasted an enormous amount of blood and treasure."

As a presidential candidate, Trump repeatedly criticized George W. Bush's and Barack Obama's handling of the Afghanistan conflict but said it would be a mistake for the U.S. to pull all troops out of the country.

"At this point, you probably have to stay because that thing will collapse about two seconds after they leave," Trump said in a CNN interview in 2015.

Less than two weeks ago, addressing the possibility of sending additional troops to the country, Trump expressed confidence in the eventual outcome but did not reveal what his administration will do there.

"It's a very big decision for me," he said on Aug. 10. "I took over a mess, and we're going to make it a lot less messy."

So far this year, 11 U.S. service members have been killed in Afghanistan. More than 2,250 Americans have died in the country since 2001.

ABC News' Luis Martinez and Elizabeth McLaughlin contributed to this report.