In your hands, my dissatisfied fellow-countrymen, and not in mine, is the momentous issue of civil war. The Government will not assail you. You can have no conflict without being yourselves the aggressors. You have no oath registered in heaven to destroy the Government, while I shall have the most solemn one to "preserve, protect, and defend it."President Abraham Lincoln, March 4, 1861 First Inaugural Address. For what reason did President Lincoln make this statement?

To communicate it to the remaining slave states that he only wants to defend Union and doesn't want to abolish slavery, which was the primary concern of the states who stayed with Union.

Explanation:

This speech was delivered amid the tensed circumstances, as the Southern states had left the Union and formed Confederacy, but few states decided to remain in Union - For whatever reasons - although they were supposed to go with the south as they were the slave states these states were Delaware, Kentucky, Maryland, and Missouri. Now the question or the issue was that what would happen to them? will they be harassed by the Union? or will Union abolish the slavery from their states forcefully? so there was a state of confusion in these states. And to clear that Lincoln made the speech and in that, he was in a quite difficult situation because he had to take a conciliatory yet a firm stance as going too rough on remaining slave states could have offended them. Further, that this statement that you mentioned was the end of his speech in which he took a very decisive position and tried to create a situation in which he wanted to show himself the rightful defender of the constitution and the Union. These lines were meant to get support from remaining states if the situation escalates to the war. So he asserted that nothing would be abolished no southerner would be harassed but the union will preserve and no one has the authority to break the Union but he does have the authority to respond and he would respond.