The dollar's gains could be a warning to global markets

The dollar gained nearly 1% against the euro on Monday and rose to its highest since early August after about two weeks of declines.

Details: The dollar index, which tracks the greenback's value against six major global currencies, also strengthened, touching its highest in six weeks.

Why it matters: The dollar was the ultimate safe haven during March's selloff that saw global funds pour into the greenback as risk assets like stocks and oil declined at a record pace.

• It reversed and moved lower as markets improved in the following months — going into Monday the currency was down nearly 5% since the start of Q3, and in July it suffered its worst month since 2010.

The big picture: The unwind of that trade could signal a broader market shift toward risk aversion.

• The dollar rose almost across the board Monday, gaining against safe havens like the Japanese yen and Swiss franc as well as touching weekslong highs against commodity-linked currencies like the Canadian and New Zealand dollars, and emerging market currencies like the Mexican peso and South African rand.