There's that comprehension problem of your's again. President Lincoln didn't go to war, he accepted the war that was initiated by the Davis regime. The forts and facilities were the property of the U.S. government and were located in what the administration believed were U.S. states and cities. Why shouldn't he want to hold on to them?
Why didn't he just accept the Southern commissioners and negotiate?
Negotiate the settlement of propety already seized by the confederacy, or that which they were preparing to bombard into submission? In other words, negotiate with a confederate gun pointed at his head. Doesn't it seem reasonable to expect negotiation before disposition of the property in question? Why didn't the confederates negotiate a settlement before they stole the property? That is, if they were serious about a fair and reasonable settlement that is.
Were their other USA military outposts in foreign lands prior to the war?
No, and there were no USA military outposts in foreign lands at the outbreak of the war, either.