In order to make a nuclear explosion, the critical mass must come together within a fraction of a millionth of a second.

为促成一次核爆炸，临界质量必须在一百万分之一秒内聚集在一起。

《柯林斯英汉双解大词典》

The pieces of the puzzle must come together at once.

分散的拼图马上能拼凑起来。

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This is the moment when we must come together to save this planet.

此刻，我们必须团结一心拯救这个星球。

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Finally, the broader global community must come together and take meaningful action against IEDs and those who use them.

CNN: Don't ignore the threat of IEDs

It is also why the world must come together to confront climate change.

CNN: Obama: 'Peace requires responsibility'

So we must come together, pass this agreement, and show our neighbors in the region that democracy leads to a better life.

NPR: Text of Bush's State of the Union Address