Copper alone is soft. Copper with about ten per cent tin is bronze, which is markedly harder, pours into a mould more cleanly, and takes and keeps an edge. The difference is large enough that the alloy names an age.
The problem with bronze is geological. Copper ore is widespread. Tin ore, essentially cassiterite, is not; it occurs in a small number of places and almost never near copper. Making bronze therefore required something no earlier technology had needed: sustained long-distance exchange between regions that had never had reason to deal with one another.
The tin came from Cornwall and Devon, from Iberia, from Brittany, from central Asia and from what is now Afghanistan. Analysis of trace elements and isotopes in Bronze Age objects has traced tin moving across thousands of kilometres. The Cassiterides, the tin islands of Greek and Roman geography, were a real trade destination whose location was a commercial secret; Herodotus admits he does not know where they are. The Uluburun shipwreck off the Turkish coast, dated to the late fourteenth century BCE, was carrying about ten tonnes of copper and roughly one tonne of tin, close to the correct ratio for bronze, which is as direct a piece of evidence for the trade as could be asked for.
The dependence cut both ways. A civilisation whose weapons and tools required an imported material from a small number of distant sources was fragile in a specific way, and the disruption of those routes is one of the standard elements in explanations of the Late Bronze Age collapse around 1200 BCE. Iron, whose ore is everywhere, carried no such vulnerability.