The issue of sovereignty is a red line that cannot be crossed.

主权问题是一条不能跨越的红线。

《牛津词典》

The red racing boat crossed the finish line first.

那条红色的赛船第一个冲过了终点线。

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So has the Syrian Government crossed a US-declared "red line" by using chemical weapons against rebel forces or civilians in Syria or not?

BBC: Is the Syrian government using chemical weapons?

In response, Senator McCain told reporters a "red line has been crossed" and recommended arming the opposition, a step the White House has been reluctant to take.

BBC: 'Growing evidence' of chemical weapons use in Syria - UK

But if a red line has been crossed, Cameron was less clear on what the next steps should be.

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