Tending the Vineyard

Luk 20:9-16 ESV
(9) And he began to tell the people this parable: "A man planted a vineyard and let it out to tenants and went into another country for a long while.
(10) When the time came, he sent a servant to the tenants, so that they would give him some of the fruit of the vineyard. But the tenants beat him and sent him away empty-handed.
(11) And he sent another servant. But they also beat and treated him shamefully, and sent him away empty-handed.
(12) And he sent yet a third. This one also they wounded and cast out.
(13) Then the owner of the vineyard said, 'What shall I do? I will send my beloved son; perhaps they will respect him.'
(14) But when the tenants saw him, they said to themselves, 'This is the heir. Let us kill him, so that the inheritance may be ours.'
(15) And they threw him out of the vineyard and killed him. What then will the owner of the vineyard do to them?
(16) He will come and destroy those tenants and give the vineyard to others." When they heard this, they said, "Surely not!"

I know that this parable is told against the Jews. However, sometimes I wonder how much it applies to our time in America and our way of life. I think we could replace the tenants in this parable with Americans. How much longer will we have a free and sovereign nation? I know that it can't go on forever because no nation does. When will it end?