It's not actually a real quote from Lincoln but there is a quote that says almost the same thing "At what point then is the approach of danger to be expected? I answer, if it ever reach us, it must spring up amongst us. It cannot come from abroad. If destruction be our lot, we must ourselves be its author and finisher. As a nation of freemen, we must live through all time, or die by suicide." -Abraham Lincoln More on reddit.com

The logistical realities of the 19th Century would’ve made it close to impossible for any European power to conquer the United States. Deploying large armies across thousands of miles of the Atlantic Ocean is a huge effort requiring massive amounts of shipping and friendly ports. Britain was the only country with enough ships and bases to have a chance of pulling it off. However, while they could theoretically conquer the United States in a purely hypothetical vacuum where they could bring the full might of the British Empire to bear, the realities of managing a global empire made it impractical. Britain’s soldiers and ships were scattered all over the world because they were needed to keep the Empire running. Any major military buildup to prepare for the war is going to draw lots of attention and probably require new taxes to pay for it all too. On top of that, Britain being distracted by a long and messy war against the United States would give all their enemies a free hand. If we presume the invasion happens in 1838 and Britain’s tied down in the US, Russia can advance in Central Asia, the Sikh Empire might start pushing into India, Belgium could get removed from the map before the Treaty of London is signed. More on reddit.com