Why is Afghanistan so crucial to the US?

[Khezar]

The US is fighting its longest war in Afghanistan, and she has no plans to leave. What, in your opinion, could be the major reason?

[Chintoo2010]

US had following reasons to prolong its stay in Afghanistan :-

Maintain its presence in the South Asian region to keep a check on China and Russia as China is emerging as global super power. Also Russia is reflexing its Military muscle so US intends to exist in the region for strategic reasons.

To check One Belt One Road vision of China


Afghanistan is adjacent to Central Asian Republics which have huge reserves of Oil and Gas and also to have access to Caspian Sea

Surely US presence is not liked by Russia and China. US Military bases in Afghanistan are a strategic gain over Russia and China.

US is presently tilted towards India as its strategic partner in South Asia. India and US nexus has many joint strategic stakes in Afghanistan. India is laying ground to access CARs throgh a netwrok of highways and railways in Iran via Chabahar port. India wants to outweigh Pakistan in Afghanistan. US sentiment is also anti-Pakistan.

On the other hand India and China do not enjoy healthy relations.
India wants to convery Afghanistan into a Military garrision to encircle Pakistan. US wants enforcement of China and also an eye on Russia besides Oil and gas reserves of CAR nations. Also US may check string of learls vision and OBOR vision of China by its presence in Afghanistan.