The message of A Passage to India is that the British imperialistic approach is not a recipe for long-term success.

Forster sees "white man's burden" ideology as a part of the British approach to India. This imperialist ideology stresses how the British have an obligation to be in India. Fielding voices this to Aziz in the final scene when he says India will crumble to pieces if the British leave it:  

Away from us, Indians go to seed at once. Look at the King Emperor High School! Look at you, forgetting your medicine and going back to charms.

Fielding did not always believe in this. However, as time has passed, he has changed to embracing a form of the "white man's burden" ideology.

Forster suggests that this approach is not going to work in India. People like Aziz will not stand for it. Aziz is very direct in the way he speaks to Fielding at the end. Aziz is insistent that Britain is going to leave India. He suggests that he is going to see it or his kids will see it. He even suggests that the time is soon at hand. Aziz argues that as England becomes involved in war it will do so without Indian support:

We used to blame you, now we blame ourselves, we grow wiser. Until England is in difficulties we keep silent, but in the next European war--aha, aha! Then is our time.

Aziz's attitude is a direct statement against the "white man's burden" ideology.

Even though Aziz and Fielding want to be friends, the social and political issues of the day prevents them from doing so.  The ending where the land physically separates Aziz and Fielding is reflective of how the "white man's burden" ideology is not going to work in India. Forster uses this ending to show that imperialism will not work in India because resistance against it is inevitable. The horse ride between Aziz and Fielding become a symbol of how India's sides are going to be drawn. As this is the ending of the novel, its message is that unless one side understands the other, friendship and success are not going happen yet.