My lord, I fear, Has forgot Britain. (1.6.133-134)
It's not just that Posthumus has forgotten Imogen—it's that he's forgotten his homeland as well. Imogen tells Iachimo she's worried that Posthumus will never come back, even if he has the chance. She's the princess, so she has to stay in Britain. Will national boundaries spell the end of their marriage?
That this will prove a war; and you shall hear The legions now in Gallia sooner landed In our not-fearing Britain than have tidings Of any penny tribute paid. (2.4.20-23)
Philario gets an earful from Posthumus, who wants to talk about whether or not Britain should pay Rome. It takes some guys to praise your country and your people when you're in the enemy's camp.
Britain's a world By itself, and we will nothing pay For wearing our own noses. (3.1.14-16)
Cloten's trying to sound all tough, but it comes out sounding pretty stupid. It sound so stupid, in fact, that it's almost as if Shakespeare is making fun of this kind of blind patriotism. Aside from the Queen, Cloten's the only one this gung-ho about the war, after all—and that's not a glowing recommendation for blind patriotism.
IMOGEN Hath Britain all the sun that shines? Day, night, Are they not but in Britain? I' th' world's volume Our Britain seems as of it, but not in 't, In a great pool a swan's nest. Prithee think There's livers out of Britain. (3.4.158-162)
Here, Imogen literally doesn't know where to go other than Britain, which is the only place she's ever lived. But she's also being metaphorical here: for the first time, she has to leave the comfort of her familiar life. It's a mental journey as much as a physical one. Both Imogen and Posthumus have to travel far from "home" in order to grow and become worthy of each other.
PISANIO These present wars shall find I love my country, Even to the note o' th' King, or I'll fall in them. (4.3.51-52)
Pisanio can see that one of the strange things about war is that it really ups people's patriotism. Even though he was suspected of offing Cloten (that would be treason), he knows he loves his country, and that's enough for him to have faith that it will all work out.
ARVIRAGUS, as Cadwal
I am ashamed To look upon the holy sun, to have The benefit of his blest beams, remaining So long a poor unknown. (4.4.48-51)
Arviragus wants to fight for his country even though he barely knows what this country is (he really just knows the cave he grew up in and the forests where he's gone hunting). It doesn't really matter to them who wins or loses the war, in the sense that his life will be pretty much the same either way. So why do you think he's really into the idea of fighting for Britain?
If in your country wars you chance to die, That is my bed too, lads, and there I'll lie. (4.4.61-62)
When Belarius decides to go with the boys to fight for Britain, we feel a pang of guilt for writing him off as just a kidnaper. It looks like he's got morals and gusto, too—and he uses it to stand by his (kidnapped) sons and his (off-limits) country.
POSTHUMUS I am brought hither Among th' Italian gentry, and to fight Against my lady's kingdom. 'Tis enough That, Britain, I have kill'd thy mistress. Peace, I'll give no wound to thee. (5.1.17-21)
When war is about to break out, Posthumus delivers a speech about whom he will fight for. He's too patriotic to fight against his country, even though he was wronged in it. We see that he cares a lot about Britain, and will risk his life trying to save it. What is less clear is whether he wants to fight for Britain just because he loves Britain, or whether he wants to fight for Britain because he's in love with a woman who (it would seem) will eventually be the queen of the country.
POSTHUMUS 'Our Britain's harts die flying, not our men. To darkness fleet souls that fly backwards. Stand, Or we are Romans and will give you that Like beasts which you shun beastly, and may save...' (5.3.27-30)
After Britain has won the war, Posthumus gets talking to some cowardly lords about the battle. Notice how he characterizes each side: it's not quite as simple as "Britain is good" and "Rome is bad." Was either side actually good or bad in this war?
SOOTHSAYER To the majestic cedar joined, whose issue Promises Britain peace and plenty. (5.5.556-557)
For years to come, Britain will prosper. That's a cool way of ending the play, at least as far as the king is concerned, so why does he agree to pay the fee to Rome, then? Cymbeline just won the war, and he knows his country will flourish. What's the deal with agreeing to the tribute? Is it to ensure peace for the future?