entangling alliances: 意思是糾纏不清/有危害的結盟
warn of: 固定短語，警告、告誡的意思
從網上幫妳找到了兩個代表性的例子：
8.In his first inaugural address, Thomas Jefferson warned of entangling alliances.
托馬斯-傑斐遜在他的就職中告誡結盟的危害。
12.Early US political leaders were wary of "entangling alliances" with other nations.
美國早期的政治領袖惟恐與別的國家結盟，使自己卷入爭端。