Ansar news agency: By looking at global developments and Long-term strategies of the United States, we will find that Washington in any way does not want to leave Afghanistan and even Iraq. Despite costing billions of dollars in the war in Afghanistan and Iraq, the United States looses a large number of its forces in there to fight with the Taliban and the Iraqi people. Therefore, due to lack of victory in fight with terrorism, the United States cannot leave Afghanistan or Iraq; in this case, the United States Army announced that it will send 2,250 fresh troops to Afghanistan and Iraq by the end of year to be replaced the old forces. Presence in Afghanistan is very important for the United States and Washington does not want to lose this opportunity anyway.  Due to the inability of the Afghan local forces, they cannot maintain security and stability of the country; therefore, the United States promotes that the withdrawal of coalition forces can lead to instability in Afghanistan and again starts a war to seize the power.
Afghanistan is important for the United States in many ways, and some of them are:
Neighboring China and Russia: Afghanistan is near to the two main rivals of the United States, China and Russia. Presence in Afghanistan means that Washington can monitor the development of the two countries closely so when it is necessary put them in trouble.
Therefore, sending this number of the United States troops to Afghanistan would be in accordance with this strategy.  Since 2010, the United States due to change its military strategy, change its focus from the West Asia and North Africa to East and South-East Asia. The purpose of this changing is limiting the power of China. No doubt, Afghanistan is a very important base for the United States in this way.
Neighboring Iran: Afghanistan is bordered by Iran, which is one of the main opponents of the policies of America in the West Asia and North Africa, therefore presence in Afghanistan makes a great opportunity to continue its Anti-Iranian plans.
The rich resources of Afghanistan: In addition to these cases should not forget the rich resources available in Afghanistan, which do not exploit yet.
 Iraq situation and its chaos, which is the result of Takfiri group Daesh activities, has become a pretext to warn about the cost of the withdrawal of the Western and the US forces, so the United States politicians use it to warn the leaders and people of West Asia and North Africa about this indirectly.
The withdrawal of Iraq makes the United States the real loser in competing with the Islamic Republic of Iran. Due to this bitter experience now, Washington will not lose its control on another country in this region to the growing power, such as Russia, China, India and even Iran.
At the beginning of 2012, it was found that the US plans to increase the number of its troops in the Persian Gulf and want to build new bases in the region, for this increased the number of its troops in Kuwait with this goal that provides its need for other bases in the region. 
In addition, France as an ally of United States strengthens its military presence in the region and since 2009 in Abu Dhabi establishes a military base, which its only aim is putting more pressure on the Islamic Republic of Iran and harness its increasing power especially in the northern waters of the Persian Gulf.
Of course, America's military overseas bases in Turkey, Israel, Djibouti and Diego Garcia in the Indian Ocean have not Rapid reaction forces and the United States tries by increasing its military abilities in Persian Gulf; use them in all probable battles.