Which allied nation could the central powers invade only by airplane?

The central powers could potentially invade the United Kingdom only by airplane during World War I, as it is an island nation separated from mainland Europe by the English Channel. Given the technological limitations of the time, large-scale air invasions were not feasible, but aerial attacks and smaller-scale operations could have been conducted using aircraft. However, the logistical challenges and the strength of the Royal Air Force would have made such an invasion exceedingly difficult.