Presidents can use the power to declare war for political purposes, i.e. start a war after a scandal, or before an election, etc. They can also be bribed, just like the politicians in congress.

The constitution grants congress the power to declare war, congress being the representatives of the people of the states. It was never intended that a single person, the president, has the authority to launch an offensive war with another country.

I agree with you that the campaigns are financed in the U.S. amounts to legal bribery. Most other civilized countries outlaw or greatly curtail campaign donations from corporations, unions, and trade associations. Many also limit donations to offices you can vote on, subsidize donations with tax credits, limit the lengths of campaigns, and/or publicly fund political parties.