They would almost certainly be able to launch a simultaneous attack to wipe out all major city centres and just level them without contaminating much of anything. that or a simple biological attack that could be delivered without us even knowing Aliens had arrived.
Also they would likely be able to take over all our computers remotely leaving us unable to control anthing requiring computer control.
I don't think that bodes well for us given the only relatable reference we have being mankind.
Using us a reference if it is the USA or Russia who finds life we can dominate and resources on an alien world they will be killed and/or enslaved and their resources claimed. If the Chinese get their first they will be eaten as some type of aphrodisiac with their resources eaten as garnish.