Simple answers follow, the germans didn't back engineer ET craft, there was no need to, their current level of technology was sufficient to 'experiment' with A/G techs, the wrights brothers didn't back engineer anything!
Its part of our natural progression, first planes then rockets........the ONLY place next to go in terms of non terra transport is ANTI GRAVITY.....no et's involved.

Now why are they keeping it from us and not using it to dominate in theatres of war??

The families/power heads seeking to dominate the globe which they see as inevitable can only do so by the creation of the ruling class and the under class, one class has access to the knowledge WE ALL should be witness to, and the underclass is actually being devolved and the only way to do this is denying access to the right knowledge/truth of what life is and what we are and how it all works.

By denying knowledge to some and not others CREATES the control system we have today!

Its no far leap from logic that the very leading edge of understanding the truth is shrouded in secrecy,murder and conspiracy.

The human race's destiny is being shaped and molded by MAN who seeks to be godlike, it doesn't have to be this way, we could ALL rule as nature intended but some it seems think they're the ONLY one.