It didn't "authorize troops into Niger".
Here's the relevant text from the June White House letter to Speaker Ryan and Majority Leader O'Connell concerning Africa operations.
"...the United States has deployed combat-equipped forces to a number of locations in the U.S. Central, European, Africa, Southern, and Pacific Commands' areas of responsibility. Such operations and deployments have been reported previously, consistent with Public Law 107-40 and the War Powers Resolution..."
snip----------------------------------------------------------
"Lake Chad Basin Region: United States military personnel in the Lake Chad Basin continue to provide a wide variety of support to African partners conducting counterterrorism operations in the region. In Niger, there are approximately 645 United States military personnel deployed to support these missions. In Cameroon, approximately 300 United States military personnel are also deployed, the bulk of whom are supporting United States airborne intelligence, surveillance, and reconnaissance operations in the region. These forces are equipped with weapons for the purpose of providing their own force protection and security, and they will remain in Cameroon, with the consent of the Government of Cameroon, until their support is no longer needed."
Statute cited by Trump administration:
https://www.gpo.gov/fdsys/pkg/PLAW-107publ40/pdf/PLAW-107publ40.pdf
Wilson's bill:
https://www.congress.gov/114/bills/hr3833/BILLS-114hr3833ih.xml
White House letter to
Republican leaders:
https://www.whitehouse.gov/the-press-office/2017/06/06/text-letter-president-speaker-house-representatives-and-president-pro
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