The festal blazes, the triumphal show, The ravish'd standard, and the captive foe, The senate's thanks, the gazette's pompous tale, With force resistless o'er the brave prevail. Such bribes the rapid Greek o'er Asia whirl'd; For such the steady Romans shook the world; For such in distant lands the Britons shine, And stain with blood the Danube or the Rhine; This pow'r has praise, that virtue scarce can warm, Till Fame supplies the universal charm.
Brave people are always greedy for honor. They want festive torchlights to be lighted in their honor, and to read stories about themselves in the papers, among other things.
The "rapid Greek" (Alexander the Great—check out "Shout Outs") conquered Asia in order to gain such honor, and the Romans shook the world for it. The British carried out successful military campaigns along the Danube and Rhine rivers (in Bavaria and Austria) for such honor.
The speaker states that "This pow'r has praise, that virtue scarce can warm." His words are somewhat ambiguous here. What he seems to be getting at is that brave, ambitious men want praise for their actions.
Virtue, or goodness, isn't a motivation for their actions. They want to be famous; they don't care about being good.
Yet Reason frowns on War's unequal game, Where wasted nations raise a single name, And mortgag'd 'states their grandsires' wreaths regret, From age to age in everlasting debt; Wreaths which at last the dear-bought right convey To rust on medals, or on stones decay.
Our reason doesn't condone war, which is essentially unfair. In war whole nations are destroyed so that one man's name can be raised up high.
In these lines we can see the literary device of personification. The speaker represents Reason as "frowning" on "War's unequal game." So Reason is described as a person, with facial expressions no less.
Nations end up in a whole lot of debt only so that the wreaths of a few "grandsires," or war heroes, can be hung up. This is regrettable. These wreaths are temporary, they decay, but that big pile of debt accumulated during war doesn't disappear.