Kabul can be taken just as easily as Mosul was because the US-trained Afghan army, like the US-trained Iraqi army, is corrupt and demoralized. They have more money and weapons than they know what to do with. What they're missing are the fundamentals like belief and will, which the Taliban, ISIL, and Shiite militias have in plenty.
An excerpt from, "$65 Billion Later, U.S. Hands the Reins to a Weak Afghan Army" by Brianna Ehley, The Fiscal Times, March 4, 2015:
A new report from the Special Inspector General for Afghanistan Reconstruction (SIGAR) contains information that the Pentagon attempted to keep secret—revealing the grim outlook on the Afghan forces.
According to SIGAR, the ANF has shrunk to its lowest level in years---with at least 20,000 troops fewer than last year.An excerpt from, "Kabul must share power with Taliban group, Musharraf says" Khaama Press, February 25, 2015:
The report comes just months after the NATO-led coalition ended its combat mission in Afghanistan. It’s grim finding raises new questions over whether the ANF will actually be able to defend the country on its own. It also calls into question whether the tens of billions used to train and retain these now dwindling troops could have been better spent
The former Pakistani President Pervez Musharraf has said that the Afghan government must share power with the Taliban militants group in a bid to bring peace and stability in the country.
Musharraf has also said that the government of Afghanistan must block Indian influence in the country if it wishes to see peace.
Speaking during an interview with The Wall Street Journal Musharraf insisted that both Pakistan and India must stay away from proxy war in Afghanistan.
Musharraf admitted that the Afghan conflict was fed by the involvement of India and Pakistan in long-running proxy war in Afghanistan.An excerpt from, "IS threatens Afghanistan peace hopes" by Jan Agha Iqbal, Asia Times, March 5, 2015:
These developments take place when militants of Islamic State (IS) are making inroads in Afghanistan and Pakistan. Al-Qaeda and the Taliban may have some rivalries with IS, but the proximity between their ideologies, goals and methodologies and tactics will bring them all under the black flag of IS.
Adding fuel to the fire, the speedy growth of the Islamic State in Afghanistan has filled some insurgents, particularly those unwilling to join the peace deal, with hope and energy to win the war. The IS has recently announced its expansion into the land of Khorasan, which mainly refers to Afghanistan and parts of Pakistan.
In Afghanistan, many among the Taliban and al-Qaeda either have pledged to IS openly or clandestinely or plan to do so, though there have been reports of clashes between Taliban fighters and IS militants. In southern Zabul and Helmand provinces, Mullah Abdul Rauf, a former Taliban commander who was recently killed, was actively recruiting fighters for IS, while in Kunar and Farah provinces the group has established training camps.
Similarly, Afghan government officials have reported about the activities of IS militants in Ghazni and Kunduz provinces in central and northern Afghanistan. The police chief of Kunduz has confirmed that 70 IS militants were operating in the province and planned to expand their activities to other provinces. Some 30 members of the Hazara ethnic community have recently been abducted on the Kabul-Kandahar highway by gunmen wearing black clothing and black masks. and believed to be IS militants.
A spokesman for the Islamic State, in an audio tape published on January 26, announced the appointment of Hafez Saeed Khan, a former commander of the Pakistani Taliban (Tahreek-Taliban Pakistan), as the "governor" of Khorasan province, and Mullah Abdul Rauf Khadi, a former senior Taliban commander in southern Afghanistan, as the deputy governor. The Islamic State in Khorasan has claimed that the group has deployed over 10,000 troops on the borders of Pakistan and Afghanistan.