The US is engaged with a war against the UFOs, not the other way round. They try and shoot them down in an attempt to recover there technology. Also they see the UFOs and there outlook on life as a threat to the economy and power structures on this planet. For example I doubt ufos have money, credit cards, debt, ect.
The only threat we face from other ET civilisations is that they wil not let us go out on masse to other planets and rape it for its resources like we do this planet. Many attitudes and beliefs that we accept as perfectly normal are frowned upon by more advanced civilisations. Like we consider slavery a great evil today, genetic engineering, animal testing, industrial farming, excessive mining of minerals and toxic pollution are not tolerated elsewhere in our universe.
What's even worse is that all the resources we extract from the earth aren't used for noble reasons. Usualy just to create products we don't need but gratify our ego temporarily.
Governments have been warned that if we attempt to go out to other planets in our current state and with our current attitudes earth will be hit with an electromagnetic weapon that will disable every electrical device on the planet and we'll be back in the stone age.