I'm working on the same question and I was able to find this conclusion;
The main reason why Abraham Lincoln helped the union win the war was because he went from limited to total warfare. Meaning he did everything in his power to help win while being practical at the same time. Thinking about what people cared about opposed to what was best. Also he valued each state and doesn't want them losing their support towards the union and by thinking politically and militarily he was able to do so in a smart way.