And why do you think it isn't now. The treaty the UN is asking us to sign now is in defiance of our constitution and will supposedly over rule it.
You seriously think they're coming after you? In white vehicles?
Vehicles seconded to UN Peace Keeping operations are painted white and troops wear blue headgear, so that both belligerents know who they are and that they're neutral. Those forces are seconded from national armies of independent countries.
They're not UN-ian troops - there's no such thing, the UN does not have its own, organic armed forces.
And if they were coming for you, they'd be in full camo and they'd be sporting some heavy armament too.
This is a British Army Warrior painted white for UN Peacekeeping operations in Bosnia in the early 90s: http://u0v052dm9wl3gxo0y3lx0u44wz.wpengine.netdna-cdn.com/wp-content/uploads/2014/06/Warrior-Bosnia-1994-Image-Credit-Cold-War-Warrior-740x471.jpg
It belongs to a British Infantry Regiment, it is manned by British infantry who answer to British commanders, notwithstanding they've been seconded to a UN force. They aren't "UN troops", they're British troops whose services are provided to the UN under a very particular UN Security Council resolution (of which the USA is a member and holds a veto power). Same holds for any UN troops you see anywhere.
Whilst with the UN, the British Army's Warriors were painted white.
When the whole Bosnia situation went completely and irredeemably to poo, the UN handed the whole debacle over to NATO which commenced Operation Deliberate Force, which involved bombs and artillery hitting the bad guys (the bad guys capitulated within 14 days) - the command of those British troops seconded to the UN was also passed (back) to NATO, so out came the paint brushes and within a few days they were all green/black and no longer UN peacekeepers and no longer to be messed with. Likewise, the UN Blue Helmet covers/berets came off and were replaced by NATO Woodland.
So, if you reckon there are "UN forces" in America, what nationality are they?
Here's a list of current UN force contributions by nationality - http://www.un.org/en/peacekeeping/resources/statistics/contributors.shtml
which of these, either singly or together has the capability to somehow mount a takeover of the US?
And given the US Army/Marine Corp/Air Force's complete and total domination in terms of number and quality of troops, equipment, professionalism and recent two-way range battle experience, what chance do you give them if they attempt to take over the country?
The only force big enough to occupy the US might, at a stretch be China, but that assumes that no one puts up any resistance and that they enlarge their navy 20-fold to include maybe 30 odd carriers and also manage to win air superiority over the homeland of the best equipped and most capable airforce and navy in the world.
My simple rule of thumb - if a rumour sounds unlikely, it's probably untrue.