"Bull markets don't die of old age" is a piece of trader wisdom trotted out to justify hanging out in stocks in a bid to try and squeeze out the last of any potential gains in a cycle. But as market lore goes, this piece of advice may now be injurious to your financial wealth.

It's true that buying on any dips as a strategy in recent years has stood investors well. The combination of central bank quantitative easing (QE) and low-bond yields has forced investors to take on more equity market risk in the hope of improving returns.

The old market trope suggests bull markets instead die of excess — too much hubris, and too much leverage. But there is one more reason: policy mistakes. Whether it is the Federal Reserve lifting rates too quickly, or President Donald Trump igniting a trade war spark that turns into a brushfire, policy risk hovers on the edge of investors' peripheral vision like a bad tempered wasp at a picnic.

There are clearly still arguments in support of equity markets. Namely, strong earnings and a moderation in share prices since the start of the year that has improved the valuation picture.

Nandini Ramakrishnan, global market strategist at J.P. Morgan, is still positive. "In the second half of the year, we expect some reacceleration of growth outside the US as… employment growth and cheap credit… reassert themselves," she said. "We expect modest gains in equity prices by year-end as government bond price drift lower." The team at J.P. Morgan still sees improvements in productivity emerging in the U.S. that will underpin markets there.

But, the escalator of fear around a tit-for-tat trade war and rising interest rates is beginning to unnerve investors.

Ralph Jainz, who manages money at Centricus Asset Management, said: "Central bank support is starting to fade, even as the global demand environment is deteriorating." He thinks investors need to start positioning themselves for a market that is rolling over: "We would expect a full-blown trade war in European equities to start in H1 2019 at the latest."