In which year did the United States formally enter World War I by declaring war on Germany?

Introduction / Context:
World War I began in 1914, but the United States did not enter the conflict immediately. Instead, it remained officially neutral for several years before finally declaring war on Germany. This question tests your recall of the exact year when the United States formally joined the war, a basic but important date in modern world history.

Given Data / Assumptions:

• The conflict known as World War I lasted from 1914 to 1918.
• The United States joined after being neutral for a time.
• The question asks specifically about the year of entry, not the year the war started.
• Only one of the given years correctly matches the declaration of war on Germany.

Concept / Approach:
The concept here is timeline recall. You should know that key events such as unrestricted submarine warfare and the Zimmermann Telegram pushed the United States toward involvement. These developments came to a head in 1917. By eliminating obviously incorrect years and focusing on the sequence of events, you can identify 1917 as the correct answer.

Step-by-Step Solution:
Step 1: Remember that World War I started in 1914, mainly as a European conflict. Step 2: Recall that the United States maintained neutrality for several years while trading with both sides to some extent. Step 3: Note that German unrestricted submarine warfare and the sinking of ships carrying United States citizens increased tensions. Step 4: Recognize that the Zimmermann Telegram, in which Germany encouraged Mexico to act against the United States, further inflamed opinion. Step 5: Know that in April 1917 the United States Congress declared war on Germany, marking the formal entry of the country into World War I. Step 6: Eliminate 1927, 1971, and 1933 as they fall outside the war years, and 1914 as the year the war began in Europe, not when the United States joined. Step 7: Therefore, select 1917 as the correct year.

Verification / Alternative check:
History books consistently list April 6, 1917, as the date when the United States declared war on Germany. They describe the period from 1917 to 1918 as the time of main United States military involvement in World War I. None of the alternative years fit the historical timeline of the conflict. This external confirmation supports 1917 as the only correct answer.

Why Other Options Are Wrong:
Option b (1927) is a postwar year associated with the interwar period. Option c (1971) falls in the Cold War era, long after World War I. Option d (1933) is linked to the rise of Adolf Hitler in Germany, not United States entry into World War I. Option e (1914) is the year the war began in Europe but does not mark the United States declaration of war.

Common Pitfalls:
Students sometimes confuse the start of the war with the entry of major powers. Another error is mixing up dates of World War I and World War II or focusing only on famous dates such as 1914 and 1939. Keeping a simple mental timeline that marks 1914 as the start of World War I and 1917 as the year the United States joined helps prevent these mistakes.

Final Answer:
1917