The Empire managed to pacify and oppress a significant chunk of the galaxy and that was a decade to nineteen years before the first Death Star was operational. So using little more than Republic Era technology and upgraded weapons and training techniques inherited from the Grand Army of the Republic the new regime blanketed most of known space with a veil of control and authoritarianism and didn't even engender an open, armed Rebellion until about 10 BBY.
So I can buy the First Order taking advantage of chaos within the New Republic and the destruction of Hosnian Prime to make blitzkrieg thrusts into New Republic space and assert control. Think of how much ground even a totalitarian military taking severe retaliatory blows can gain when it launches a major offensive during a lull in the other side's judgment? Nazi Germany was on its last legs in December of 1944 yet still managed to launch the invasion of the Ardennes and the Battle of the Bulge, costing the Allies many lives and resources and bogging American, British and other troops down for weeks. A wounded animal can be a dangerous one and still kill.
So I can buy the First Order taking advantage of chaos within the New Republic and the destruction of Hosnian Prime to make blitzkrieg thrusts into New Republic space and assert control. Think of how much ground even a totalitarian military taking severe retaliatory blows can gain when it launches a major offensive during a lull in the other side's judgment? Nazi Germany was on its last legs in December of 1944 yet still managed to launch the invasion of the Ardennes and the Battle of the Bulge, costing the Allies many lives and resources and bogging American, British and other troops down for weeks. A wounded animal can be a dangerous one and still kill.