。
The enemies behind us are, for now, just there to cut off our retreat — nothing more.
Literal
Rear [gen-の] enemy [topic-は] current-state [just-あくまで] escape-route [obj-を] cut-off only [gen-の] existence.
Military/strategy analysis. 後方 ('rear') contrasts with 前方 ('front'). あくまで ('just, merely, to the end') functions here as a limitation marker — 'only to the extent that, nothing more than.' 退路 ('retreat route') is a formal military compound. 断つ ('to sever, to cut off') is a slightly literary verb for decisive severing. ~だけの存在 ('an existence (that is) only for X') uses だけの attributively to describe a limited role. The whole sentence is dense with formal/military vocabulary, sketching a tactical assessment.