What was lincoln greatest challenge upon being inaugurated as the 16 president of the us?

Lincoln's greatest challenge upon being inaugurated as the 16 president of the US was keeping the Union together, since many southern states believed that this presidency and administration would threaten slavery and thus their economy and way of life. 

In my honest opinion , i’d say franklin d roosevelt was a president that impacted our nation , i say this because fdr is voted among top three best us presidents and he was the longest serving president by 3 terms , usually presidents only supposed to have two terms increments of 4 years , equivalent to 8 years total before election time. and he established the fdic , through the 1933 banking act and he established the us , social security system , during his presidency, and established a national minimum wage , through the fair labor standards act , fdr led the allied coalition to victory in wwii and he played a major role in the creation of the united nations.