i would interpret 將 in that sentence as "in near future" (即將).
mencius challenged 齊宣王, said "or (抑), your majestic (王) increases (興) armours (甲) and soldier (兵), [puts] your officials (士臣) in risk (危), makes up (構) enmities (怨) amongst (於) dukes (諸侯). and then (然後) [you'll be] cheerful (快) in (於) [your] heart (心)?"
his majestic (王) said (曰) "no (否). how come (何) i (吾) [would be] cheerful (快) for (於) these (是)? in near future (將), [these policies are] for (以) seeking (求) my (吾) greatest desire (大欲)"
I do (those things) to seek what I greatly desire!
quite good, as a shortcut :)
well, 齊宣王's greatest desire is in the following text:
. . . 欲辟土地．朝秦楚．莅中國而撫四夷也
[your] desire (欲) [is]: open up (辟 --> 開闢) [more] land (土地), [have representatives] in the court (朝 --> 朝廷), from the nation of 秦 and 楚, administrated (莅 --> 蒞) the china (中國), and (而) soothe (撫) all surrounding barbarians (四夷).
at that time (bc 319-301), 齊 is a small nation in 山東半島. then, "中國" is "關中平原".