Wouldn't it be easier if the essay you were reading had a title? If it did, you would have a good idea from the start what the main point of the essay is. The writers of the GMAT purposefully exclude the title so that it is up to you to decipher the essay and its big idea.

Most of the GMAT questions, particularly higher skill level questions, aren't about details, they concern the main idea. The tone, scope, and implications of the main idea usually help you answer more than half of the questions on a given passage. The main idea is the Rosetta stone of a passage, helping us decipher the passage and discern its structure. Accordingly, we must focus our strategy on easily finding the author's point of view and main idea.

In nearly all GMAT passages, the author will be making an argument of some form. Don't expect the main point of a passage to be "World War I was fought from 1914 to 1919.” Instead, it's more likely to be "World War I was extended by Britain's needless and poorly executed intervention".

3b. Long Essays

5. Diagnose Author's Purpose