What analogy does pyle use to communicate the idea that the most people at home have a incomplete comprehension of what war is like?

Pyle's analogy states that "it is like seeing the trailer of a movie and saying you've seen the whole picture."  

Ernie Pyle was a journalist who became a war correspondent during World War II, embedded with the soldiers and seeing what they saw at the battlefront.  He told the story of the war from the vantage point of the fighting men -- what he called the "worm's eye" view of the war rather than the "Big Picture."  Ernie Pyle won a Pulitzer Prize in 1944 for his stories of ordinary soldiers in the war.  In 1945, he was hit by enemy fire and killed while with the soldiers on Iejema, a small island near Okinawa, Japan.  In 1983 (posthumously), Pyle was awarded a Purple Heart for his valor as a civilian correspondent of the war.

Pyle explains that people at home watching war reports is like watching a trailer for a movie. They have only seen pieces, but think that they know "everything" that is happening. The only people to truly understand and "see" the entire movie of war are those involved. Only the soldiers truly understand all the aspects of war. Civilians can attempt to empathize and understand where they are coming from, but they will never truly understand because they were not there. They didn't live it. 

Political forces often influence public opinion.