Jamie Dimon doesn't waste words, and his latest warning to investors cuts through the noise: an escalating conflict with Iran could reignite inflation and force the Federal Reserve to keep interest rates elevated far longer than markets currently expect.The JPMorgan Chase CEO laid out the scenario in his annual shareholder letter, a closely watched document that functions as a state-of-the-union address for global finance. Dimon's track record for calling major economic shifts gives his warnings significant weight.Here's the transmission mechanism that matters: War in the Middle East disrupts oil supplies. Oil prices spike. Higher energy costs flow through to transportation, manufacturing, and virtually every other sector of the economy. Headline inflation jumps. Core inflation, which strips out volatile energy prices, soon follows as businesses pass through higher costs.The Federal Reserve then faces an impossible choice: cut rates to support a weakening economy, or keep them high to combat resurgent inflation. Given the Fed's mandate and institutional memory of 1970s stagflation, they'll choose fighting inflation. That means rates stay "higher for longer."Markets haven't fully priced this risk. The futures market currently expects the Fed to cut rates multiple times in 2026. But if oil prices spike above $100 per barrel and stay there, those rate cuts won't happen. Bond yields will climb. Mortgage rates will remain elevated. Corporate borrowing costs will stay high.Dimon has been consistently more hawkish on rates than consensus forecasts, and he's been consistently right. When Wall Street was predicting rate cuts in early 2024, he warned they'd come later and slower than expected. He was correct.The current situation in Iran adds a new dimension of risk. Unlike previous Middle East conflicts that were relatively contained, a broader war involving Iran could disrupt shipping through the Strait of Hormuz, through which roughly 20% of global oil supplies flow. That's not a tail risk—it's a central scenario if the conflict escalates.Dimon also noted that persistent inflation isn't just about energy. Labor markets remain tight in many sectors. Government spending continues to run hot. Structural factors like reshoring of manufacturing and geopolitical fragmentation of supply chains are inherently inflationary.For businesses, the implications are clear: plan for a higher-rate environment. Companies that loaded up on cheap debt during the zero-rate era need to refinance at much higher costs. Capital-intensive industries face margin pressure. Commercial real estate, already stressed, faces another headwind.For consumers, higher-for-longer means mortgage rates stay elevated, suppressing home affordability. Auto loans remain expensive. Credit card interest rates stay punitive. The wealth effect from rising home and stock prices that drove consumption in the 2010s works in reverse.For investors, Dimon's warning suggests defensive positioning. Financial stocks benefit from higher rates, but most other sectors face margin compression. Long-duration growth stocks get hit hardest by higher discount rates. Value and quality outperform in this environment.Wall Street wants to believe the best-case scenario: a quick resolution to Middle East tensions, inflation continuing to moderate, and the Fed engineering a soft landing with gentle rate cuts. Jamie Dimon is telling them to prepare for the alternative. Given his track record, investors would be wise to listen.