Answer:
1. The US first sent troops into Afghanistan in October 2001.
2. The troop level at its highest was in 2011, when there were over 100,000 US troops in the country.
3. As of 2017, there are around 8,400 US troops still in Afghanistan.
4. It is so hard to defeat terrorism because it is an ideology that is fueled by a complex mix of factors, including political, economic, and social grievances. It is also difficult to target and eliminate terrorist networks, as they are often spread across multiple countries and regions.