(CNN) Like two US Presidents before him, Donald Trump has a plan to win the United States' longest-running war .

In a long-awaited speech this month, Trump made it clear the US will remain a major actor in Afghanistan for years to come -- doubling down on the conflict with a likely troop increase -- but he also called on other countries in South Asia to do their part.

Once a key battlefield in the " Great Game " between major world powers in the 19th century, Afghanistan today is a mess of overlapping foreign policy and security interests for regional and international powers.

Who are the key players?

United States

The US invaded Afghanistan on October 7, 2001, after the Bush administration accused the country's then Taliban government of sheltering al Qaeda leader Osama bin Laden, who masterminded the September 11 terrorist attacks.

The Taliban offered to hand over bin Laden for trial, but only to a third country, rather than directly to the United States. Washington refused the offer and launched air and ground attacks, joined shortly thereafter by US allies.

US troop deployment to Afghanistan peaked in August 2010 at 100,000. At present there are around 9,000 US soldiers in the country, along with smaller detachments of NATO troops.

The 16-year war and reconstruction effort has cost the US upwards of $841 billion and perhaps into the trillions of dollars by some estimates.

Despite initially supporting a US withdrawal from Afghanistan, Trump seemed to indicate last week he will approve another troop increase, but refused to give detailed figures, saying "America's enemies must never know our plans, or believe they can wait us out. I will not say when we are going to attack, but attack we will."

Pakistan

In a statement Thursday, Pakistan said it rejected Trump's "allegations and insinuations" and accused him of "scapegoating" the country for problems in Afghanistan.

Pakistan -- a major US non-NATO ally -- borders Afghanistan and has long been a key route for US supplies into the country. It has also, according to US and Afghan officials, been a major route for terrorist groups and funding, particularly the Haqqani Network

Like many aspects of Pakistan foreign policy, its actions in Afghanistan are shaped by relations with its rival India.

Islamabad fears an Indian-dominated or closely allied Afghanistan would leave it encircled and vulnerable in any potential conflict, according to a report by the Rand Corporation

"Islamabad seeks a weak Kabul government dominated by a pliant, supportive Taliban so that Pakistan can maintain 'strategic depth' against an Indian invasion, guarantee safe haven for Islamist proxies that it supports, prevent Delhi from projecting power in South Asia, and obstruct India's ability to support separatists in the Pakistani province of Balochistan," the report said.

"So long as India is viewed as an existential threat, and so long as the military plays a central role in setting Pakistani policy, it is unlikely that there will be a fundamental shift in this policy bias."

Pakistan has also been a major victim of terrorism and the instability brought about in its neighboring country by the US invasion. Islamabad also argues its investment and aid in Afghanistan is overlooked.

While there has long been tension between Kabul and Islamabad -- President Ashraf Ghani said last year the Taliban would not survive a month without Pakistani support -- Pakistan is nevertheless one of Afghanistan's biggest trading partners, according to MIT data , importing more than $392 million worth of Afghan goods in 2015.

Sen. Mushahid Hussain, chairman of the Pakistan Senate Defense Committee, wrote for CNN this week the country is "an abiding and favorite scapegoat for US failures in Afghanistan."

India

As he admonished Pakistan for not doing enough to tackle terror in Afghanistan, Trump also invited Islamabad's key regional rival to increase its presence in the country.

Since Afghanistan acts as a sanctuary for terrorist groups which have attacked India and Indian interests in the past, seeking to stabilize the country and increase its influence there makes sense for Delhi.

India is a major trading partner for Afghanistan and is increasing investments in the country. Delhi is also the biggest regional donor to Afghanistan and fifth largest donor globally with over $3 billion in assistance since 2001, according to former Afghan ambassador to India Shaida Mohammad Abdali.

"India is poised to go from soft power to hard power (in Afghanistan)," Sten Rynning, a professor of international security and war studies at the University of Southern Denmark, wrote following Trump's announcement

"India is investing more in Afghanistan than Pakistan is, including by offering more funds for development, yet to date it has kept its political profile low and restrained its export of arms."

While Delhi has provided training for the Afghan army and other security personnel since 2011 , Trump seemed to indicate he would expect global partners to take a more active role in future on matters beyond aid and economic support.

However, critics of greater Indian involvement warn it could increase tensions and even potential conflicts with Pakistan, which accuses Delhi of fomenting violence in its border regions with Afghanistan.

"Greater Indian involvement will play into fears of strategic encirclement in Pakistan," Uzair Younus, an analyst at Albright Stonebridge Group, warned last week, while Hussain said it was a "sure recipe for a Pakistan-India proxy war on Afghanistan soil."

China

Greater Indian involvement may also increase tensions with China, which has increased its footprint in Afghanistan in recent years along with shoring up relations with Pakistan.

Relations between India and China have become strained in recent months amid an ongoing territorial dispute in the Himalayas . Afghanistan shares a border with China.

In a statement Tuesday following Trump's speech, Chinese Foreign Ministry spokesperson Hua Chunying said "the international community should fully recognize Pakistan's anti-terrorism efforts" and appreciate its "important contributions to safeguarding regional and global peace and stability."

Beijing-Kabul ties have also increased significantly, with Afghan President Ghani choosing China as his first overseas visit in 2014

China is Afghanistan's third-largest trading partner, with bilateral trade reaching over $1 billion by 2015, according to the Central Statistics Office of Afghanistan. Beijing is also investing upwards of $46 billion in the China-Pakistan Economic Corridor (CPEC), as part of its One Belt, One Road (OBOR) trading initiative, which China has called on Afghanistan to be a major player in.

"China's ambitious project of regional connectivity through Central Asia is conditioned on sustained stability in Afghanistan," Carnegie India analyst Arushi Kumar said earlier this year

In June , Chinese Foreign Minister Wang Yi said Beijing hoped to "further deepen bilateral cooperation in anti-terrorism and security" with Kabul. China has also urged Afghanistan to improve ties with its ally Pakistan.

Russia

Like the US after it and Britain before it, Russia knows the costs of overlong military engagement in Afghanistan.

Moscow has long considered Afghanistan within its sphere of influence, and in 1979 the Soviet Union invaded the country, plunging itself into a decade-long war that massively destabilized the region and likely helped contribute to the collapse of the Soviet Union itself.

The US helped back an insurgency against Moscow's forces, a move that may have inadvertently empowered Islamist extremists groups like the Taliban, which would takeover the country follow the Soviet Union's exit.

While Russia no longer shares a border with Afghanistan, it has legitimate fears instability in the region, particularly the spread of ISIS could hurt its interests.

"Russia has increased its contacts with the Taliban and provided limited support out of concern that US military forces may withdraw from the region; as part of a broader strategy to increase Russian influence across the globe; and to weaken Islamic State," Rand Corporation analyst Seth Jones told the US House Foreign Affairs Committee in April

According to a recent report by the Carnegie Institute, Russia's goals in Afghanistan have put it largely in alignment with China and Pakistan.