London (CNN Business) 1. Trade war: The 2019 market rally has been impressive. Now it faces a big obstacle: trade tensions.

The resumption of the trade war between the United States and China has surprised investors and hit global stocks. The S&P 500 shed 2.2% and the Dow fell 2.1% for the week after President Donald Trump followed through on his pledge to hike tariffs on Chinese goods worth $200 billion. China has said it will retaliate, though both sides are still engaged in negotiations.

More than any other factor, trade has knocked back US stocks from recent record highs and has the potential to hurt stocks even more

"The big issue for the market right now is clearly the trade issue," said Ed Yardeni, president of investment advisory firm Yardeni Research.

Back in December — the last time volatility was a major discussion — the market was sent into a panic over slowing economic growth, a hawkish Federal Reserve and unease of US-China relations.

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