The US banned the slave trade in 1807 and our navy, like the British, did a lot to put the remaining slavers out of business. Whatever illegal slave trading was going on out of Northern or Southern ports was rather small scale. Given that Africa was still largely unexplored, there was nothing that we could do about slavery in Africa (or in Asia or Arabia). Nor would anyone then have expected us too.
And yet, I'm not sure that "The North" did become "indifferent to the worldwide problem of slavery." There were abolitionist and colonization organizations and activities. And certainly there was more concern about putting slavery behind us in the North than in the South. Beyond stopping slave ships at sea, we weren't going to have human rights interventions around the world, but more people in the North were trying to set our house in order when it came to slavery than in the South.
If you're persistent, it's in ducking the question that slavery was the South's problem to deal with in mid-century America. Southern elites failed to deal with it. They failed to hold on to their power over the country. They failed to make their separation from the country work. It's not that they lost the war. It's that they failed to achieve their goals without war, and pushed the country into civil war.
So stop being a pissant and deal with that. Why this failure to cope with what everyone could see was an explosive situation?
The trouble with avoiding war is that the enemy gets a vote too, and Lincoln voted that he would have a war. This is what happens when you send a fleet of warships to subjugate people who do not want to be subjugated. They tend to get shooty.