Question

What was the main reason that anti imperialist league opposed the war between us troops and Philippines rebels

Answer 1

The main reason the league opposed the war is because they believed that the war was immoral and undemocratic.

Answer 2

Answer:

The league believed the war was immoral and undemocratic.

Historical context/details:

The Anti-Imperialist League was established in 1898.  They viewed the United States' involvement in the Philippines was an act of imperialism.  

The central philosophical argument of the Anti-Imperialist League was that a republican government (such as the United States) cannot be true to itself if it takes over control of territories without the consent of the people in those territories.  In our own country's Declaration of Independence, you'll find these words:  "To secure these rights (life, liberty and the pursuit of happiness), Governments are instituted among Men, deriving their just powers from the consent of the governed."  If we believe that about our own country, we need to apply the same principle to other countries -- such as the Philippines.   

That was the Anti-Imperialist League's contention.