If you are going to invade neutrals you need a big reward(such as Middle East for Axis and the abillity the to invade Europe by land e.g. US attacking Spain) otherwise it is very detrimental.
As for invading the US…more people joke about it than really try but here is a way that has worked for me( keep in mind that I, my opponents, and teammates were not very skilled when I did this; believe or not I lost a game holding Washington once):
Turn one and two threaten sea lion while building a substantial navy of warships and transports. This keeps UK in check: Russo is in a quandary; US builds navy. T3 dependiing upon the US buys (most of the time you can’t to this) move to Gibraltar. T4 attack US coast and East US. This is a very little chance. It works better in Global if the US is concentrating elsewhere. US has two easy ways to stop you: either stack Washington or build a big enough Navy to rival yours. It really only works if US has little defence and you haven a substantial navy. Once when US build 4 bombers among other units, it worked due to their lack of defense.
I consider this to always be an option but most players will know khow to prevent this.
