Can the subject of a sentence also be the direct object?

Nothing answers the question so the sentence has no direct object. The direct object must be a noun or pronoun. A direct object will never be in a prepositional phrase. ... Source: Lesson 101, nor does it have a linking verb as a predicate nominative sentences does.

Explanation:

although world war i began in 1914, the united states did not join the war until 1917. the additional firepower, resources, and soldiers of the u.s. to tip the balance of the war in favor of the allies. remaining neutral. when war broke out in 1914, the united states had a policy of neutrality.