Worries about a nuclear escalation to the Russia-Ukraine war rattled markets, stealing focus from Nvidia and Walmart earnings.

U.S. stocks are drifting in mixed trading on Tuesday after clawing back losses from earlier in the morning. The S&P 500 was virtually unchanged, as of 11 a.m. Eastern time, after erasing an early drop of 0.

This Fed-based market signal is flashing a warning for the first time in over a decade. Here’s why it matters.