What is the BEST example of checks and balances in action?

The best example of checks and balances is that the president can veto any bill passed by Congress, but a two-thirds vote in Congress can override the veto. Other examples include: The House of Representatives has sole power of impeachment, but the Senate has all power to try any impeachment.

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first, the romans stayed isolated because they had to defend their own territories from neighboring cities that were harassing them, like the etruscans. second, once the romans managed to defeat their neighbors, and started to expand their territory, they started to build roads and bridges. finally, now that the romans have covered their territory with roads functioning as highways, they could mobilize their legions faster to assemble larger military units in a specific place.