When the first of what would be collectively called the four wars for empire between France and England erupted in 1689, the mainland of North America was not as important to either nation's ambitions as it would be in 1763, when the fourth war ended. Only in that last conflict would North America be the strategic focus for either power. Balance of power considerations lay at the heart of each conflict. In the early 1650s both France and Spain had considered an alliance with England under the commonwealth, despite the republicanism there that they found so anathema. The principal immediate postwar cost in North America would be the posting of troops on the frontier to enforce the Proclamation Line. Those troops were expected to prevent white settlers from moving west, to remove those who had taken up residence across the line, and to block unlicensed merchants from trading with the tribes there.