The Setup
In October 1990 the United Kingdom joined the European Exchange Rate Mechanism — the ERM, a system committing member currencies to trade within agreed bands against one another, as a stepping stone toward a single currency.
Britain joined at a rate many economists considered too high, and at an awkward moment. Germany had just reunified, and the Bundesbank was running high interest rates to contain the inflationary cost of absorbing East Germany. The UK, meanwhile, was sliding into recession and needed *lower* rates.
That is the whole problem in one sentence: the ERM required Britain to defend a currency level that its own economy could not support.
To hold sterling inside its band, the UK had to keep rates high enough to make holding pounds attractive. To help its economy, it needed to cut. It could not do both, and everyone could see it.
What a Currency Peg Actually Costs to Defend
When a currency is falling toward the bottom of its band, a central bank has three tools:
1. Buy its own currency using foreign exchange reserves. This works only while reserves last.
2. Raise interest rates, making the currency more attractive to hold. This works only while the economy can bear it.
3. Talk — signal resolve and hope speculators believe it. This works only while the first two remain credible.
Each is finite, and a speculator's job is to estimate how finite. The asymmetry is what makes the trade attractive: if the peg holds, the speculator loses a modest amount on financing costs. If it breaks, the currency moves a long way, quickly.
The Trade
George Soros's Quantum Fund had been positioning against sterling. After public comments from German officials suggesting some ERM currencies might need to realign, Soros — on the reading of his own team, notably Stanley Druckenmiller — concluded the situation was not a probability but a near-certainty, and scaled the position from around $1.5 billion to roughly $10 billion.
The mechanic was straightforward. Borrow sterling, sell it for Deutschmarks, wait. If sterling devalues, buy it back cheaper, repay the loan, keep the difference.
He was not alone. Once the direction became consensus, a great many funds and corporate treasuries did versions of the same thing, and that collective selling is what actually overwhelmed the defence. The story is told as one man against a central bank because that is a better story.
16 September 1992
The Bank of England spent an estimated 40% of its foreign exchange reserves buying pounds. It did not hold the rate.
The government then raised the base rate from 10% to 12%, and announced a further rise to 15% later the same day. Both were attempts to make holding sterling irresistible.
Neither worked, and the reason is instructive: the market did not believe the 15% would ever take effect. A rate that would devastate mortgage holders in a recession is not a credible commitment, and a commitment nobody believes provides no defence. The second rise was never implemented.
That evening the Chancellor, Norman Lamont, announced that the UK was suspending its membership of the ERM. Sterling fell sharply. Soros's fund is reported to have made around £1 billion.
What It Actually Teaches
Reserves are finite, conviction is not. A central bank defending a level has a countable amount of ammunition. The market's willingness to press does not have a comparable limit.
A promise the market does not believe is not a defence. The 15% announcement is the cleanest example in modern finance of a policy failing because it was not credible, rather than because it was wrong on paper.
One-way bets attract crowds. The trade was not a secret. Its very obviousness is what made it work, and that is a general property: pegs die when defending them is visibly against the defender's own interest.
The aftermath was not what anyone predicted. Freed from the ERM, the UK cut rates, sterling found a lower level, and the economy recovered faster than most forecasts allowed. "Black Wednesday" has been reassessed by many economists as the day an unsustainable policy ended. That is worth holding onto: the market outcome and the political framing are different things.
What Happened To The ERM
The mechanism did not die that day. It survived, with the bands widened dramatically in 1993 — to ±15%, which is close to admitting that narrow bands could not be defended against determined capital flows.
It then did the job it had been designed for. The ERM was always a staging post toward monetary union, and in 1999 the euro was introduced, with notes and coins following in 2002. Members that adopted it removed the problem permanently by removing the exchange rates: there is no speculative attack available against a currency that no longer exists separately.
Britain did not join. The domestic political reading of Black Wednesday made that outcome close to inevitable, which is a reminder that currency events have consequences far beyond the price. A successor arrangement, ERM II, still operates as the waiting room for countries preparing to adopt the euro.
The structural lesson survives the specific institutions. A fixed exchange rate, free movement of capital and an independent monetary policy cannot all be had at once — the "impossible trinity". The ERM tried to hold all three and discovered which one gives way first. Any peg you see today, anywhere in the world, is managing that same trade-off, and can break the same way.
The Part Relevant To a Retail Account
It is tempting to read this as a template. It is closer to a warning.
Soros's fund had multi-billion-dollar balance-sheet capacity, direct information channels, and — critically — the ability to survive being early. Most retail traders attempting a version of this trade lose not because the direction is wrong but because they cannot fund the position long enough for the direction to matter.
Central bank interventions and peg breaks are also precisely when spreads widen, liquidity thins and orders fill nowhere near the price on screen. That is not a broker being unfair; it is what a market with no bids looks like. The Swiss franc in 2015 is what happens to a retail account when it goes the other way.
If you are trading around scheduled central bank events, understanding your broker's execution and margin policy matters more than the direction call. Our guide to what FCA regulation actually means covers the protections that decide what happens when a move outruns your margin.
Capital at risk. CFDs and leveraged forex are complex instruments and a majority of retail accounts lose money trading them. This is general information, not financial advice, and nothing here suggests any position.