The twin-engine Chinook helicopters spotted over Gerasdorf near Vienna on Monday almost certainly belong to the US Army. Since May 4th, the NATO partners and their armed forces have been training as part of a large US-led military exercise called “Defender Europe 21”. The locations are primarily training grounds in Southeastern European countries such as Albania, Croatia, Bulgaria and Bosnia-Herzegovina – and to get there, the transport helicopters also fly over Austria. With permission, of course, troops and equipment will be transferred via Austria until mid-June.