For decades Germany was allergic to debt. But new chancellor Friedrich Merz has - unexpectedly - loosened the country’s constitutional debt brake, injecting hundreds of billions of euros into the armed forces and infrastructure. The move, he hopes, will revive Europe’s largest economy and build up its military as Donald Trump’s US administration dismantles the transatlantic relations that underpinned Germany’s postwar recovery. The FT travels to Frankfurt and Berlin to examine why investment in crumbling schools, roads and rail infrastructure - and defence - is needed and to ask if the spending gamble will kickstart Germany's economic engine.