CEO of Apple Tim Cook attends China Development Forum 2019 at the Diaoyutai State Guesthouse on March 23, 2019 in Beijing, China.

Apple closed down nearly 6% on Monday after news of a major escalation in the U.S.-China trade war.

China said on Monday that it decided to raise tariffs on some U.S. goods after President Donald Trump threatened to further raise tariffs on Chinese imports last week.

The trade war is affecting a lot of different stocks, but Apple seems to be hit harder than most. The Dow Jones Industrial index dropped 2.6%, and the Nasdaq Composite dropped 3.5%.

Apple is especially vulnerable to a trade war with China for two primary reasons.

First, it assembles its iPhones primarily in China. Although it has a lot of American suppliers — it spent $60 billion on American suppliers in 2018 — iPhone assembly is done in mainland China.

Whenever new tariffs are announced, investors must keep an eye on the details because it's possible that some of Apple's products could get caught in the crossfire.