The air war eats up more than two-thirds of the cost of the entire anti-ISIS campaign.

空中战争的花费占据了整个打击ISIS活动花费的逾三分之二。

(voice-over): A sentiment echoed by commanders leading the air war against ISIS.

（画外音）：领导对伊斯兰国进行空战的指挥官们也表达了同样的看法。

And this is all recognition, I think, that the US feels it has to intensify the battle against Islamic State, that the air war is not enough.

美方对加强打击力度有了清醒的认识，空袭远远不够。

A sentiment echoed by commanders leading the air war against ISIS. We can't take our eye off the ball. It is -- it is ISIS. That's why we're here.

领导与 ISIS 空战的指挥官们也表达了同样的看法。我们不能把目光从球上移开。它是——它是 ISIS。这就是我们来这里的原因。

Above the trenches, the first air war continues to escalate. Each side now has more than 3,000 aircraft in service on the Western Front. But by 1918 the Allies have won air superiority, thanks to greater resources.

沟壕之上，第一次空战也在持续升温。两边都有多于3000架飞行载具服役。但至1918年，协约军已经掌控了空域优势，全因有了更多的资源。