《战国策·西周·秦召周君》
西周国君不去秦国，秦国一定会担心西周将断绝它的后路。便不敢渡过黄河来进攻南阳。 
周君不入秦，秦必不敢越河而攻南阳。 
If King Western Zhou did not go to Qin, Qin would definitely worry that Western Zhou would cut off its rear, so they would not dare to cross the Yellow River to attack Nanyang.